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1. The Anterior Or Cephalic Aorta
Turning forward from the truncus, the anterior aorta runs just beneath the pericardium, directly alongside the oesophagus, to enter the cavity of the trunk on the left side (Text-Fig. 4, Fig. I, A Ao 1) immediately above the columellar muscle. It continues forward for about half the distance to the head, close to the left side of the oesophagus, and then crosses obliquely above the oesophagus to run along the right side of the gut as far as the nerve ring, through which it passes immediately below the oesophagus. It then breaks up at a single point into the proboscideal (Pro A) and pedal arteries (Ped A) and gives rise in addition to several smaller branches.
Before entering the trunk, the anterior aorta gives rise to several branches, of which the superficial gastric artery (S Gast A) is by far the largest, passing directly backwards along the left side of the visceral mass to branch extensively upon the dorsal wall of the style sac and stomach. About half way along the style sac. this vessel sends forward a more slender branch, the anterior artery of the style sac, (A St S), while the main trunk continues backwards over the stomach.
The stomach in Struthiolaria forms a large flask-shaped sac from which the proximal limb of the intestine and the crystalline style sac lead forward together, being visible from the external surface as far as the posterior limit of the pericardium. This region of the alimentary canal is richly supplied with blood, and is invested by a zone of connective tissue, constituting a phagocytic depot, which is functionally a part of the digestive system. (Text-fig. 2.) Beneath the epithelium of the anterior part of the stomach, the connective tissue forms a dense meshwork, supporting huge numbers of amoebocytic cells or phagocytes of the same type as those found in the blood stream. This layer of phagocytes extends forward to surround the proximal limb of the intestine; here it becomes deeper than elsewhere and lies immediately beneath the epithelium. Around the style sac it is separated from direct contact with the base of the epithelium by a layer of much less dense connective tissue, filled with large, branching or spherical, black pigment cells. This zone of pigmented cells is clearly visible
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Text-Fig. 3.—Schematic summary of the course of the venous system in Struthiolaria, and of the relationships of the principal veins and sinuses.
externally, and forms a shield-shaped area overlying the whole exposed surface of the style sac, and sending back a salient on to the wall of the stomach. Around the pigmented zone, the investment of phagocytic connective tissue is cut across tangentially by the body wall, so that it appears as a narrow greyish-white line, delimiting the shield-shaped area of pigment. (Text-fig. 1, Phag 1) The pigmented zone is very richly vascularised from the superficial gastric artery and from its main branch, the anterior artery of the style sac. Both vessels contribute to a rich, closely anastomosing arterial plexus; and at its periphery, this plexus comes in close contact with the sheath of phagocytes. This is the locality in which these cells migrate in large numbers out of the blood stream and become aggregated in the connective tissue. Whether division or proliferation of phagocytic cells occurs here, or whether there is any reverse migration into the blood system, is still uncertain. The writer has already spoken of the role of the phagocytes in digestion (Morton, 1951). They probably form both an accessory ingesting system in the stomach and a means of excretion in the intestine. The phagocytic depot surrounding the intestine seems to merge directly into that of the stomach and the style sac, and it is apparently supplied also by several longitudinal intestinal arteries, running beneath the intestinal epithelium and arising from the posterior aorta near its origin from the heart. Similar depots
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Text-Fig. 4.—Fig. 1—General view of the arterial system. The pallial cavity has been opened by the removal of its dorsal wall, bearing the gill and hypobranchial gland, and the visceral mass has been somewhat displaced from its normal relations. The haemocoele has been opened by a median longitudinal incision of the trunk, which has been carried forwards along the dorsal surface of the head and along the proboscis. The arteries of the foot are represented as by transparency. Fig. 2—The arterial system of the visceral mass and of the rectum and genital ducts (female), viewed from the right side. A Ao I, Point of entry of the anterior aorta into the trunk. A Gen D, Artery of the genital duct. A Mid I, Artery of the middle intestine. A Ped A, Anterior pedal artery. A Rect G A, Anterior rectogenital Artery. A Stg A, Anterior artery of the sorting area. A St S, Anterior artery of the style sac. Hep A, Smaller hepatic arteries. H Rect A, Hepatorectal artery. Integ A, Integumentary arteries. L Liv, Left lobe of the digestive gland. L Oes A, Longitudinal oesophageal artery. Oesoph, Oesophagus. Oes A 1, Oesophageal artery. Oes A 2, Oesophageal arteries within the trunk. Op A, Optic artery. Pall A, Pallial Artery. Ped A, Pedal artery. Pen A, Penial artery. Pro A, Proboscideal artery. P Stg A, Posterior Artery of the sorting area. P Subcut A, Posterior subcutaneous artery P. Teg Pro, Posterior tegumentary artery of the proboscis. P Tent A, Pallial tentacular Artery. Rect, Rectum. Rect A, Rectal branch of anterior rectogenital artery. R Fd G. A, Right artery of the food groove. R Liv, Right lobe of the digestive gland. S Gang A, Artery of the supraintestinal ganglion. S Gast A, Superficial gastric artery. Tent A, Tentacular artery. Visc A, Visceral artery or posterior aorta.
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of phagocytic cells have been found to occur in Lunella smaragda (Turbinidae) in the connective tissue surrounding the posterior sorting caecum of the stomach (Morton—unpublished observation), and in numerous other prosobranchs. There seems frequently to be a close association of these phagocytes with the ciliated sorting areas of the stomach, which are usually placed in close vascular contact with the phagocytic depots. Thus, for example, in Struthiolaria, the greater part of the ciliary sorting area upon the dorsal wall of the stomach is supplied by the anterior artery of the sorting area (A Stg A), a branch of the superficial gastric artery, which runs along the posterior limit of the phagocytic depot, and gives off to the sorting area numerous close-set parallel branches, following the course of the ciliated grooves and ridges. From examination of sections the sorting area would appear to be seldom a region of dense storage of phagocytes; but it seems to be without doubt the site where the phagocytes may take up solid particles from the lumen of the stomach, probably after migrating between the epithelial cells. After the transparent veliger larvae of Struthiolaria had been fed with particles of finely divided neutral red, a coloured area was almost at once developed in this part of the wall of the stomach. A similar process of ingestion by phagocytes was observed in the posterior caecum of the stomach in Lunella smaragda; while an earlier description of ingestion by phagocytes in the sorting area is that of Yonge (1923) on the bivalve Mya.
In Struthiolaria, the more posterior part of the sorting area is supplied from the left side by a series of small arteries similar to those from the anterior artery of the sorting area, but running in the opposite direction. These arteries arise as small, parallel branches from the posterior or visceral aorta.
Shortly in front of the superficial gastric artery, the anterior aorta produces a series of three or four small oesophageal arteries, (Oes A 1), as well as a slender integumentary artery. (Integ A) supplying the external body wall adjacent to the pericardium.
In the course of its passage through the trunk, the anterior aorta gives rise to the following series of arteries:—
| (i) |
Posterior integumentary artery of the trunk (P Integ A), a slender vessel crossing above the oesophagus to pass through the muscular body wall to the outer integument of the trunk. |
| (ii) |
Oesophageal arteries (Oes A 2), a series of tiny vessels arising at regular intervals and breaking up finely upon the wall of the middle region of the oesophagus. Further forward, just behind the nerve ring, arises a slender longitudinal oesophageal artery (L Oes A) which runs backwards along the dorsal surface of the oesophagus as far as the posterior end of the trunk. |
| (iii) |
Posterior subcutaneous artery (P Subcut A). This is a narrow but rather important vessel, springing from the aorta just after it crosses the oesophagus. It passes directly through the musculature of the dorsal wall of the trunk, to emerge on the outer surface, where it branches extensively just beneath the epithelium of the floor of the pallial cavity. Its most important branch is a prominent longitudinal artery, the right artery of the food groove (R Fd G. A) which runs backwards in the fold of epithelium that forms the right margin of this groove. This vessel conveys the main supply of blood to the wall of the food groove which—like all the mucus-secreting regions in Struthiolaria—is extremely well |
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vascularised. The rich supply of blood enables the great expansion in breadth of the bounding fold, which, together with the fold of the other side, can be extended across the food groove to close it as a temporary tube. |
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| (iv) |
Supraintestinal ganglionic artery (S Gang A). Some distance in front of the origin of (iii), a very slender vessel leaves the aorta on the left side, passes beneath the oesophagus and runs backwards along the left pleuroparietal connective. It supplies a fine plexus of blood vessels lying within the sheath of connective tissue surrounding the subintestinal ganglion which, like the nerve ring itself, is richly supplied with blood. Behind the ganglion, a very narrow artery continues backwards for some distance along the left parietovisceral connective. |
| (v) |
Pallial Artery (Pall A). The supply of blood to the mantle skirt has taken on an increased importance in Struthiolaria. With the modification of the gill as an organ of food collection, the margin of the mantle has become the chief respiratory area; as well, its connective tissues are richly supplied with amoebocytes carrying the calcium salts required for shell secretion. The pallial artery is a stout trunk arising from the aorta on the left some distance behind the point where the oesophagus is crossed. It plunges through the muscles of the trunk at the level of the supra-intestinal ganglion. Within the mantle it divides into anterior and posterior branches which run around the skirt and become continuous again on the right to constitute a complete circum-pallial arterial loop. The pallial artery gives off numerous, close-set arteries which break up into fine parallel branches running towards the free margin of the pallial skirt. In addition, there are somewhat stouter branches running to the siphonal lappet on the left and to the pallial tentacle on the right (P Tent A). The edge of the mantle is very extensible, especially the posterior pallial skirt, which forms a thickened cushion over the calloused inner lip of the shell. Here the arterial vessels break up into a dense plexus from which blood is returned to the pallial venous loop. |
After passing through the nerve ring, below the oesophagus but above the short zygoneury (Pl. IV Zyg), the anterior aorta divides into the pedal and proboscideal arteries (Ped A, Pro A), both very stout trunks serving to pour a rapid supply of blood into the pedal and proboscideal sinuses respectively. At the same point, separate and paired tentacular (Tent A) and optic arteries (Op A) arise, running directly through the body wall to the cephalic tentacles and the optic peduncles. In the male, a very stout penial artery (Pen A) originates on the right side. It carries a large blood supply, by which the penis is able to be elongated and distended.
The pedal artery continues as a stout median trunk which afer a short distance bifurcates into two lateral pedal arteries running backwards along the sides of the foot, deep to the sole. Each sends off, near its origin, a much shorter anterior pedal artery (A Ped A) which breaks up in the pedal gland along the anterior margin of the foot. The whole cavity of the foot consists of a large haemocoelic blood space, traversed from side to side by slender muscle strands and able to be quickly flooded with a large volume of blood. In the typical creeping movement by which the animal progresses, expansion of the foot is chiefly secured in this way. When the operculum is in use, for gaining purchase in the substratum (see
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Morton, 1951), there is little or no blood in the pedal sinus; the foot is reduced to a narrow muscular column, and the sole diminished in size by the expulsion of blood into the cephalic sinus and the haemocoele of the trunk. The pedal arteries do not branch further and appear to ramify very little at their final distribution; blood is evidently poured into the pedal sinus through wide arterial trunks ending rather abruptly.
The proboscideal artery produces branches of the same abruptly ending type. It passes forwards along the floor of the proboscis immediately below the oesophagus, and remains very wide. It carries a large volume of blood to the very extensible proboscis, which may be erected in the same way as a lamelli-branch siphon and is used as a piston for constructing the inhalant and exhalant tubes in the sand, and for periodically clearing them of obstruction. By extension of its longitudinal muscles, the proboscis may be elongated and pushed up to the surface of the substratum. As it is drawn back it is greatly widened so as to fashion the wall of the sand tube. By the flooding of its sinus with blood, the proboscis reaches three or four times its normal diameter, and sometimes increases to several times its normal length. Clearly its supply of blood is required to be rapid and efficient. This is in part assisted by the relaxing of the circular muscles of the wall of the proboscis, and is chiefly brought about by the action of slips of extrinsic muscle originating from the muscles of the wall of the proboscis, and inserted radially upon the connective tissue sheath of the proboscideal artery itself. The lumen of this vessel is thus able to be suddenly dilated, and blood is poured into the haemocoele of the proboscis from the termination of the proboscideal artery.
Along its whole length, the proboscideal artery gives off also a series of tiny lateral arteries, which extend dorsally around the wall of the oesophagus and branch minutely. In addition there is a pair of stout posterior tegumentary arteries of the proboscis (P Teg Pro) arising about half way along the proboscis, and passing obliquely forwards and outwards to the muscular wall Reaching the buccal bulb, the proboscideal artery gives rise to a pair of slender salivary arteries running upwards to the dorsal surface of the bulb. Just behind the radular caecum, the main trunk bifurcates, and from each branch comes a series of arteries supplying the expansible integument of the oral disc.