214 E. W. SEXTON. 



Jassa pusilla, G. O. Sars. 



(Stebbing (41), pp. 655 and 739.) 



Station YII. One specimen, a large ovigerous female, 5 5 mm. long. 



„ XIII. Three specimens, 2 males, 5 and 5*5 mm. respectively, and 

 1 female, 4*75 mm. 



The separation of this species from /. falcata (Mont.) is rendered 

 difficult not only by their great similarity, but by the enormous range 

 of variation found in falcata, which at a first glance suggests the possi- 

 bility of 2^usilla being only a young stage of that species. Walker 

 was inclined to consider them identical ; he says (45), p. 314: " If it be 

 admitted that Amphipoda may become sexually mature before they 

 have attained their final moult, I think these species [pusilhis and 

 Kcrdmani] can hardly be maintained " ; and again (44), p. 473 : " I am 

 disposed to consider F. Iferdmani and F. pusillns (Sars) as examples of 

 arrested development and mere varieties of F. falcatns." 



Sars and Stebbing, on the other hand, consider them distinct species, 

 but, as several of the characters given by them for distinguishing the 

 one from the other are tho.se subject to developmental modifications, I 

 have thought well to discuss the different points in detail. 



The two most useful and constant distinguishing features will be 

 found in the second gnathopod, viz. the second sideplate and the hand. 

 The inferior margin of this sideplate in intsilla forms a continuous line 

 with the margins of the 1st and 3rd sideplates, its anterior margin 

 beino- as long as the posterior margin of the 1st. In falcata, on the 

 contrary, the anterior margin is much shorter than the posterior mar- 

 gin of the preceding sideplate, only half the length in full-grown speci- 

 mens, oiving a curious and characteristic appearance to the animal, as 

 if the head and 1st peraeon-segment were divided from the rest of the 

 body (the coloration adding to the effect, the head of the 1st segment 

 and sideplates being invariably darkly pigmented, and the 2nd seg- 

 ment and sideplates light with a few patclies of pigment). This char- 

 acter is found even in the young in the incubatory pouch, though with 

 them the anterior portion of the sideplate is rounded, whereas in the 

 full-grown animal it is angular (see figs. 10 and 11 for comparison). 



With regard to the second distinguishing feature, the hand, the 

 adult male of jJ^^silla has the apex of the thumb bifid ; in the adult 

 male of falcata it is entire ; and an examination of many thousands of 

 specimens of this latter species shows that in it tlie characteristic bifid 

 form of imsilla is never met with. 



General aspect. The body is more compressed in pusilla, the side- 

 X^lates longer in proportion, and the peraeopods longer and more 



