VI NEPHROPSIDEA CRAYFISHES — ^ERYONIDEA I 57 



exopodites of the thoracic limbs, and is converted into the young- 

 Lobster, measuring about half an inch in length. The little 

 Lobster starts in deepish water, and gradually crawls towards 

 the shore ; here it passes its adolescence, but on coming to 

 maturity it migrates out again izito the deep water. 



Fam. 2. Astacidae. — In this family, which includes all the 

 European and North American Crayfishes, Astacus (Fotamobivs) 

 and Cambarus, the podobranchs are united with the epipodites, 

 the last thoracic segment is free, there is only one pleurobranch 

 or none at all, the gills have a central lamina, but the filaments 

 are without terminal hooks, and the endopodites of the first 

 two pairs of abdominal appendages in the male serve as 

 copulatory organs. For the distribution, etc., of these forms 

 see p. 213. 



Fam. 3. Parastacidae. — This family includes the Crayfishes 

 of the Southern Hemisphere, viz. Parastacus from South America, 

 Astacopsis and JEngaeus from Australia, Paranephrops from New 

 Zealand, and Astacoides from Madagascar. These genera agree 

 with the Potamobiidae in the union of the podobranchs with the 

 epipodites, and in the free condition of the last thoracic segment, 

 but there are generally four pleurobranchs, the gills are without 

 a lamina, the filaments have terminal hooks, and there are no 

 sexual appendages in the male. For distribution, etc., see 

 also p. 213. 



The larval development in the Crayfishes is still more abbre- 

 viated than in the Lobsters, the Mysis stage being passed through 

 within the egg-membranes. The young, when they hatch out, 

 are furnished with hooks upon the chelipedes, by which they 

 anchor themselves to the pleopods of the mother. 



Tribe 2. Eryonidea. 



These are remarkably archaic animals of great rarity, though 

 they were common enough in Triassic seas, and have come down 

 to us as fossils from those times, being thus among the oldest 

 Decapoda known. They only survive now as deep sea species, and 

 the genus discovered by the Challenger} Willemoesia (Fig. 105), 

 confirmed the expectations of the Challenger naturalists that the 

 abysses of the ocean would contain relics from older periods which 



' Challenger Reports, x.xiv., 1888. 



