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Volume 37, 1904
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Art. XXVI.—On the Occurrence of a Species of Cercaria in the Cockle.

By Chas. Chilton, M.A., D.Sc., F.L.S., Professor of Biology at Canterbury College.

[Read before the Philosophical Institute of Canterbury, 30th November, 1904.]

Plate XIX.

During the month of November, 1904, I spent a short holiday at the Portobello Fish-hatchery and Biological Station, and on the morning after my arrival the curator, Mr. T. Anderton, brought me a cockle (Chione stutchburyi) evidently infested with some parasite. On examination this proved to be the sporocysts of some species of Distomum containing numerous Cercarice, in most cases just ready to escape from the sporocyst. Subsequently the sporocysts were found in two or three other specimens of the cockle, but, though numerous in those specimens in which they were found, they were not often met with—certainly not in 1 per cent, of the cockles examined.

Similar Cercarice have been long known to occur in various molluscs in Europe, but so far as I am aware the only forms hitherto recorded from New Zealand are the two described by Professor Haswell from Mytilus latus;* and, as my specimens differ from these and from all the others that I can find descriptions of, I give here a brief description of its general form, mode of occurrence, &c. It appears to come very near to Cercaria villoti, Monticelli (= C. setifera, Villot),† but differs from that

[Footnote] * Proc. Linn. Soc. N.S.W., 1902, p. 497.

[Footnote] † See Korscheldt and Heider, “Embryology of Invertebrates,” part i. (English edition), p. 186; and Villot, “Annales des Sciences Naturelles,” Zool., viii., p. 33. (I am indebted to Professor Benham for these references.

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species in the character and arrangement of the setse on the tail, and I propose to give it the name Cercaria pectinata.

Cercaria pectinata, n. sp. Plate XIX.

Similar to C. villoti, Monticelli (= G. setifera, Villot), but with the “setse” arranged in two rows along the sides of the tail, instead of forming rings around it; each “seta” consisting of a short vertical row of long bristles.

Hob. In Chione stutchburyi, Otago Harbour.

General Notes.

The sporocysts (fig. 1) were numerous in each cockle in which they were found, particularly in the first one, in which there must have been several hundreds. They were lying external to the body in the spaces between the mantle-lobes, the gills, &c. Each sporocyst is about 4 mm. long and 0.75 mm. broad, sausageshaped, but somewhat narrowed at each end, both ends being alike. By alternate contractions and expansions they were capable of creeping about somewhat slowly with a worm-like motion. The whole of the interior of each sporocyst was in nearly all cases filled with fully developed Cercarice just ready to escape, and through the semi-transparent walls of the sporocysts these Cercarice could be seen often moving about within. The number of Cercarice in each sporocyst varies to some extent, but usually there were a dozen or more. Swimming among the sporocysts were numerous Cercarice that had already escaped, and if a sporocyst were artificially ruptured the contained Cercarice at once began to swim freely about. In a few cases the Cercarice were seen escaping naturally through the walls of the sporocyst, and in these cases they made their way through the lateral wall and were apparently able to penetrate this at practically any point.

Occasionally the sporocysts contained, besides fully developed Cercarice one or two rounded or elongated masses of cells, presumably immature Cercarice, but in most of the cases observed they contained nothing but fully developed Cercarice.

The Cercaria (fig. 2) has a long powerful tail nearly twice as long as the head; this tail sometimes showed numerous fine transverse lines or striae, most evident anteriorly, but these were not made out in all cases. Along each side of the tail is a series of “setae,” and by the rapid vibrating action of the tail, combined with the use of these “setae” as fins, the Cercaria is able to swim rapidly through the water. When viewed from above during life each “seta” (see fig. 3) appears to be simple, but closer observation shows that each is made up of a number of fine long bristles arranged in a short vertical row and united together at the base so as to form a comb-like structure, the

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teeth of which are, however, very long. These bristles are held together by fine transverse connections, and when in action during life are kept in the one plane and moved as a single structure, but in preserved specimens they become separated and spread out towards the distal end. The tail is also capable of considerable contraction and expansion, and its proportionate length and width accordingly varies greatly, but in the condition most usually maintained the width is about one-eighth of the length.

The head is usually more or less rounded when the Cercaria is swimming freely, but in addition to this mode of motion the Cercaria can creep along by means of the worm-like motions of the head caused by contraction and expansion, and accordingly the head then assumes other shapes and may be much elongated. In fig. 2 the anterior portion is shown in the act of contracting and consequently widening, while the posterior portion is still elongated. The general colour of the whole Cercaria is whitish, semitranslucent, but on the head there are usually a number of small irregular pigment-spots of a light-brown colour.

The general structure of the Cercaria appears to present the usual features: the buccal sucker is situated at the anterior extremity, and is the foremost portion when that part of the body is being extended; the posterior sucker is about equal in size to the anterior and is situated a little anterior to the middle of the head; in fig. 2 it appears rather nearer the anterior end owing to the contraction of that portion of the head. There is a well marked cuticle, and around the anterior sucker this bears a number of fine short setae. The internal organs are-well developed; during life the most conspicuous is a large V-shaped structure extending from the posterior extremity of the head nearly to the anterior end, and presumably representing the contractile bladder of the excretory system; in life this is darker than the rest of the head, and is filled with minute granules. The central portion of the tail presents much the same appearance, but whether the bladder actually extends into the tail or not I am not certain.

The alimentary system is also well developed and of the usual structure; the pharynx is small, nearly circular, and situated close up to the buccal sucker; the oesophagus narrow and short, and the two lateral branches of the intestine are also narrow.

The reproductive organs also appear to be well developed in most of the Cercarice examined, and in one which was accidentally ruptured while under examination I saw mature spermatozoa; in some the genital aperture could be seen about midway between the two suckers, and connected with it an oval structure extending back as far as the posterior sucker and apparently representing the rudiment of the genital atrium. I have, however, not

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attempted to make out the arrangement of the different parts of the reproductive system. In specimens killed in osinic acid two small masses staining deeply with the acid are usually very prominent, one on each side, a little behind the posterior sucker.

As the Cercarice on leaving the sporocyst may readily escape from the cockle and swim freely in the sea-water, it may thus reach a great variety of animals, and in the meantime I am unable to offer any suggestion as to the host in which the adult fluke is likely to be found.

Explanation of Plate XIX
Cercana pectinata.
Fig. 1.

Sporocyst containing Cercarice (x about 10).
Fig. 2.

Cercaria pectinata (× 60).
Fig. 3.

Cercaria Pectinata: one of the “setca” from the tail (highly magnified).
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