Genus Oxycephalus, Edw.
Milne-Edwards, Ann. des Sc. Nat., t. xx., 1830, p. 396.
C. Claus, Die Gattungen und Arten der Platysceliden, 1879, p. 44.
Body elongated, slender, cephalon produced into a triangular beak, from the base of the under-surface of which the anterior antennæ project. These have the peduncle greatly dilated in the male, and thickly furnished with olfactory setæ; flagellum 2- or 3-jointed. The posterior antennæ are 5-jointed in the male, and lie behind the snout under the inflated portion of the head, all the joints being folded close against one another; in the female they are wanting. The mandibles are small, and furnished with a slender 3-jointed palp in the male. The maxillæ are totally wanting. The maxillipedes are also greatly reduced in size, and their squamiform plates are smoothly rounded. The gnathopoda have complex chelæ, the carpus being produced into a long narrow point, which meets the dactylos; those of the first pair are shorter than the second. The first two pairs of pereiopoda have the joints very slender, in the third and fourth the basa are broadly dilated, while the fifth pair are very much smaller, but have all the joints present. The three posterior pairs of pleopoda are double-branched, the branches being broadly lanceolate. The triangular telson seems to be anchylosed to the preceding segment.
Spence Bate, in the British Museum Catalogue of the Amphipodous Crustacea, has overlooked the remarkable sexual differences which characterize the Oxycephalidæ, not only in this genus, but also in Rhabdosoma, in which he describes the male of R. armatum as a separate species, R. whitei. The distinctive characters have been clearly brought out by Dr. Claus in a paper on “Die Gattungen und Arten der Platysceliden,” which however is not readily accessible to New Zealand students. In this paper, Dr. Claus describes Oxycephalus piscator, Edw., at considerable length, and unites O. tuberculatus, Sp. Bate, and O. oceanicus, Guérin-Méneville, to it—the latter being a young male. He also gives brief descriptions of six new species, from all of which the following species is quite distinct, though apparently nearest O. latirostris, a Lagos species. The brevity of the descriptions however, and the want of illustrations, render this resemblance somewhat doubtful.
1. Oxycephalus edwardsii, n. sp. Plate xii., figs. 14–21; pl. xiii., fig. 1.
Male.—The head is widely dilated and produced into a long sharp snout. This snout is more or less sharply ridged on the upper surface, and nearly flat on the under-side, the margins being sharply bent inwards. The sides of the head are nearly completely occupied with the eyes, which resemble those of Phronima. The sides of the head are not in close contact below, but form a long groove in which the posterior antennæ lie folded. The anterior antennæ are placed in front of the head just under the base of the beak; they depend nearly vertically, and have their concave side turned outwards. The peduncle has two short basal joints, and then a long, very stout, curved joint, the whole inner (convex) surface of which is thickly coated with olfactory setæ. The flagellum, which projects nearly at right angles from the extremity of the peduncle, is very small and 3-jointed.
The posterior antennæ are placed almost behind the head, and their joints lie folded closely together in the groove under the cephalon, in a zigzag manner; when extended, they are two or three times as long as the head and snout. Dr. Claus calls these organs 5-jointed; they have 4 long, subequal joints, which are extremely slender, but a little dilated at their ends, with a minute terminal hook-like claw, which appears only to be present in mature males. The mandibles are much reduced in size, and project down, behind the insertion of the antennæ, as small tooth-like organs furnished with a slender 3-jointed palp. These and the very much reduced maxillipedes are the only mouth-organs present; and the latter are of very simple structure, consisting each of an oval smooth plate, without any trace of hairs or teeth. The gnathopoda are relatively small, and the first pair are only about half the size of the second. In both pairs the basos is elongated, and the carpus produced on its inferior margin into a
long spine, against which the dactylos impinges. In the first pair the carpus is short and stout, its inner surface, as well as that of the propodos, is furnished with a considerable number of short stiff setæ or spines. In the second pair both carpus and propodos are elongated, and their finely-serrated palms are almost destitute of setæ. The 1st and 2nd pereiopoda are long, slender, and sparingly furnished with hairs. The 3rd pair have the basa finely-serrated on the lower front margin, while posteriorly they are dilated into an oblong plate; they are quite naked. In the 4th pair, the basa are dilated into very broadly pear-shaped plates, while the remainder of the limb is finely fringed with pectinate setæ on the front margin. The last pair are small, and have the basa slenderly pear-shaped. The basa of these three pairs act as protective shields to the side of the body, and the remaining joints of the limb when at rest lie folded up under them. The three anterior pairs of pleopoda or swimmerets, have an oblong basal joint, with two finely-setose branches. The three posterior pairs are also double-branched, but are of very unequal length. The first pair have the peduncle more than twice as long as the branches, both of which are movable, and the inner one of which extends to the extremity of the telson. The second pair only extend to the end of the peduncle of the first, and have the outer joint alone movable. The third pair, which are placed at the extremity of the last body-segment are also short, reaching to the end of the telson, and having the outer joint alone movable. In all the pairs the branches are finely-serrated on the margins, and the movable outer one is always smaller than the inner. The telson is elongated, and sharply pointed; its separation from the last joint of the abdomen can be made out on the ventral surface somewhat imperfectly, but from above it seems to be completely anchylosed. The abdominal segments are all produced into a sharp spine posteriorly.
Female.—The sexual differences are very considerable in these animals, showing themselves almost exclusively in the cephalon and its appendages.
The head is much more inflated than in the male, being nearly globular, so that the beak is more prominently shown; the sides of the head appear to be completely fused together below, so that there is no groove for the antennæ as in the male. The anterior antennæ are much simpler than in the male: the first joint is tolerably long, the second very short, while the third is also straight and very much more slender than the male, while only a few olfactory setæ are developed on its outer margin; the flagellum is 2-jointed, but the last joint in mature specimens appears to be sometimes divided into two. The posterior antennæ are quite absent, as are also the mandibular palps. The length of the body is from 1 to 1 ¼ inch. The animal is absolutely transparent and glass-like.
Hab. I found numerous individuals washed up on the Ocean Beach near Dunedin on two different occasions. They appear to come ashore in fine clear calm weather.
|Figs. 1–8.||Corophium excavatum.|
|1. Adult; 2. mandible; 3. second gnathopod; 4. third pereiopod; 5, 6 and 7. ante-penult., penult., and ultimate pleopoda; 8. telson.|
|Figs. 9–10.||Edotia dilatata.|
|9. Adult female; 10. opercular plate.|
|Figs. 11–13.||Pseudæga punctata.|
|11. Dorsal view; 12. lateral view; 13. head, seen from the front.|
|Figs. 14–21.||Oxycephalus edwardsii.|
|14. Adult male; 15. anterior antenna, female; 16. anterior antenna, young male; 17. posterior antenna, young male; 18. mandible-palp; 19. first gnathopoda; 20. second gnathopod; 21. crystalline cones of the lateral eyes.|
|Fig. 1.||Oxycephalus edwardsii.|
|Posterior portion of abdomen, showing telson and three posterior pairs of pleopoda; also the structure of the extremity of the intestine.|
|Figs. 2–5.||Allorchestes recens.|
|2 and 3. female and male anterior gnathopoda; 4 and 5. female and male posterior gnathopoda.|