224 



Arthropoda. Class 1. Crustacea. 



developed caudal fin, in others (Crabs)^ whicli have a feeble abdomen^ it 

 is entirely wanting. The adult Reptantia move on the sea-bottom by 

 means of their strong ambulatory legs (which in Prawns are of quite 

 subordinate locomotor importance)^ whilst they are incapable of 

 actually swimming;* those which have a muscular abdomen can shoot 

 backwards like the Prawns. 



1. The Lobster {Homarus vulgaris) is a large dark -blue Cnistaoean with 

 a very musotilar tail and wide fin ; second antenna, with exopod, and a long, 

 powerful flagellum. The first pair of ambulatory legs are strong chelse, of whicli 

 one (sometimes the right, sometimes the left) is the larger, and is beset with more 

 inob-Uke teeth than the other ; the second and third pairs are also chelate, but 



Fig. 185. 



Fig. 186. 



Fig. 185. Quite a yonng Lobster-larva (Mysis-stage), dorsal and lateral views, 

 enlarged. — After Sara. 



Fig. 186. — ^Newly hatched Cray-fish, enlarged. — ^After Huxley. 



are no stronger than the remaining two pairs. The Lobster does not pass 

 through a zosea stage, but at hatching is aheady provided with all the 

 ambulatory hmbs, which, like the thii-d maxilhped, bear swimming rami, by 

 means of which the almost transparent animal rows itseK through the water. 

 The mysis-stage is followed by a prawn-stage, from which, finally, the perfect 

 animal emerges. Common on European coasts, especially on the coast of Noi-way ; 

 an allied species is caught in quantities on the coast of North America. 



2. The Cray-fish {Astacus fiuviatilis) is in most respects hke the 

 Lobster (three pahs of chelae, etc.), but differs from it, among other things, in 

 that the body is smaller and somewhat thicker, that the large chelae of the first 

 pair are equal, and that the feeler of the second antenna is shorter and weaker. 

 With reference to the development, the Cray-fish behaves very differently, 

 not only from the Lobster, but also from almost all Decapods. When the young 

 one leaves the egg it is aheady in most respects hke the adult animal; in 

 particular, all the ambulatory legs are nearly as weU developed, and have no 



* Some Crabs, in which the last pair of thoracic appendages is much flattened, 

 execute by their means a sort of swimming movement. 



