MICROSCOPICAL STUDIES. 65 



number of somites composing this part of the body. But several 

 weighty reasons point to the eyes as not representing a somite, and 

 this reducing the number to 13, we find the entire body to consist of 

 20 somites. And this number not only characterises the Decapods, to 

 which the Lobster belongs, but equally the whole of the higher 

 Crustacea. In some species, indeed, the thorax is distinctly jointed 

 after the fashion of the abdomen, emphasizing clearly the primitive 

 segmentation of the former region. 



In the Lobster as in all Decapods, the breathing organs consist 

 of lancet shaped gills composed of innumerable closely packed plates 

 or lamella?, connected with the ambulatory limbs and some of the 

 maxillipeds, and lodged in a special champer on either side, formed 

 by the down-turned edges of the carapace. 



If we now turn to Squilla, we have to note the following radical 

 differences from the Decapod type. Most are apparent in the accom- 

 panying drawing of Squilla mantis, the larger of the two British 

 species. 



This animal is much more active and lithe than the Lobster, by 

 reason of the fusion of the head and thorax being less complete. 

 The carapace leaves uncovered the last three somites of the thorax, 

 and the shortened ones bearing the hinder maxillipeds are also not 

 fused with this shield. Apparently then, Squilla shows a more 

 primitive form of structure than does the Lobster — for undoubtedly 

 the architypal crustacean had a body composed of a number of 

 somites of which none were fused together — all being separate and 

 independent. As to the appendages, the eyes are extremely curious 

 in shape, expanding at the summit in a broad bilobed fashion that 

 arrests the attention at once. Then follow two pairs of antennas, the 

 outer bearing a broad oval scale or squame ; next a pair of strong 

 mandibles and two pairs of maxillse, just as in the Lobster. But 

 here the similitude ends, for instead of three pairs of foot-jaws, there 

 are five pairs, — the first weak, the second formed into strong and 

 greatly developed prehensile claws, — with toothed terminal joint 

 capable of folding down into a groove on the inner side of the 

 joint preceding, while the succeeding three are weakly organs, 

 with rounded penultimate joint, bearing a tiny spine-like claw. 

 Next, instead of the five pairs of walking limbs characteristic 

 of Decapods, there are in Squilla but three pairs, and in struc- 

 ture they differ also, being weak and styliform and bearing a 

 delicate outer branch (exopodite), which disappears entirely in the 

 Decapods — one of several reasons stamping the latter group as more 

 modified, more distant, from the primitive co-parent, than the 

 Mantis Shrimps. As to the abdomen however, the form of the 

 appendages remains essentially the same, saving that in Squilla the 



