226 



Arthropoda. Glass 1, Crustacea. 



very small and assist in holding the animal in its sheU ; the fifth have also the 

 work of cleaning out the gill-cavity* into which they are introduced from behind. 

 The Hermit-crabs hatch as z o bb se and pass directly from this condition to the 

 prawn-state, ia which they swim about by means of the abdominal appendages ; 

 the abdomen at this time is muscular and perfectly symmetrical. At the con- 

 clusion of this stage the animal seeks a small empty shell which it later exchanges 

 for others of a gradually iacreasiug size. Hermit-crabs are found in all seas. 

 P. Bernhcurdus lives in the North Sea and in the English Channel, and lodges in 

 the sheU of the Whelk. 



5. Crabs (Brachyv/ra) are a division of the Decapoda, consisting of 

 many genera, and very rich in species, foi-ming, as it were, the summit of 

 this sub-order; for on the one hand the perfection of the posterior 

 thoracic legs as ambulatory organs, on the other, t h e reduction 

 of the abdomen, here reaches its climax. The body is broad (the 

 cephalothorax frequently wider than long), the abdomen is much flattened, 

 short, and feeble, and is turned up on the ventral side of the thorax; in the 

 female it is wider than in the male. The antennse are short, the second antenna 

 has no exopod ; the last maxillipeds are laminate, covering the other mouth parts 

 like folding-doors. Only the first pair of ambulatory appendages are chelate, 

 the rest are strong walking legs. The sixth pair of abdominal legs (those 

 of the caudal fin) are wanting; in the female the eggs are carried by the 

 second to the fifth (the first pair is, as a rule, missing) ; in the male, only the first 

 and second pairs, which form the copulatory organs, are present. The Crabs 

 hatch as zosese with the first and second maxillipeds developed as natatory 

 organs (the third pau' is not developed in this way) ; the crab-zoaea is often 

 characterised by having long spines on its short carapace. There is no mysis- 

 stage, but the young one passes through a prawn-stage (the so-called megalops), 

 in which it is in most respects like the adult animal, but the more powerful 

 abdomen is backwardly directed, and is provided with appendages which act 

 as swimmerets. Finally the abdomen and its appendages become reduced, the 



Fig. 188. A, zoseaof a, Crab, B — C prawn-stage from above and from the side, 

 enlarged (in C the walking legs are for the most part out off). A^, A^ first and second 

 antennse, 1 — 3 maxillipeds, B5 last abdominal leg, T dorsal spine. — After Bathke. 



* In some Prawns (probably many) the first, somewhat feeble, ambulatory legs are 

 put to the same use. They are pushed into the cavity from in front and below, and 

 brxish over the ^ills to clean them. 



