VI NEPHROPSIDEA——CRAYFISHES—ERYONIDEA 157 
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 into the deep water. 
Fam. 2. Astacidae.—In this family, which includes all the 
European and North American Crayfishes, Astacus (Potamobius) 
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 filameuts 
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 Engaeus trom 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, ete., 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 
1 Challenger Reports, xxiv., 1888. 
