372 



THE INVERTEBRATA 



pairs are present. After its first moult, which takes place almost 

 at once, it has a median dorsal spine. The latter two features are 

 characteristic of the Zoaea of the crabs. A Megalopa larva (Fig. 277 B), 

 with the cephalothorax crab -like but the abdomen macrurous and 

 carried at length, intervenes, as in other crabs, between the Zoaea 

 and the adult form. 



an', 

 -an'.' 



Fig, 277. Larval stages of Carcinus maenas. A, Zoaea. After Faxon. 'R^ Me- 

 galopa. After Bate, an . antennule; an", antenna; d.sp. dorsal spine; md. 

 mandible; jnx'-mx" . maxillae; mxp'-mxp" . maxillipeds; rst. rostrum. 



Of the various examples of the order which are mentioned below, 

 all except Leucifer, Birgus^ and Gecarcinus occur in British waters. 



The most aberrant member of the Decapoda is the minute, pelagic 

 Leucifer, which has a very slender, macrurous body with an extremely 

 elongate head, long eyestalks, no hmbs on the last two thoracic 

 somites, no chelae, and no gills. Like the normally built prawn 

 Penaeus and the rest of the group (Penaeidea) to which both belong, 

 Leucifer starts life as a Naiiplius. 



Leander, the common prawn, one of the Caridea ; macrurous like 

 the crayfish, but built for swimming rather than walking, with 

 phyllobranchiae, and with chelae only on the first two pairs of legs. 



Crangofi, the shrimp, is related to Leander but has a broader and 

 flatter body, a very small rostrum, and the first leg subchelate. 



Nephrops, the Norway lobster, one of the Astacura, differs from 

 the crayfish in minor points, among others in having the podobranchs 

 free from the mastigobranchs. 



