380 



ARTHROPODA 



(fig. 402) these all flow together in a common mass, connected with the 

 brain by two long oesophageal commissures. 



The development of most decapods is interesting from the number of larval 

 forms. As a rule a zoea (fig. 379) is hatched from the egg; this passes next into 

 a Mysis stage (fig. 403) in which head, thorax, and abdomen are distinct, the 

 thorax bearing biramous feet like those of schizopods a proof of the origin of 

 the simple feet from the biramous type. In the crabs (Brachyura) the Mysis 

 stage is replaced by a Megalops (fig. 404), in which the abdomen is well de- 

 veloped, but the feet have lost their biramous character. In some prawns^ 



FIG. 401. FIG. 402. 



FIG. 401. Reproductive organs of (.4) female and (B) male crayfish (from 

 Huxley), od, oviduct; od', its opening on nth appendage; <n<, ovary; t, testes; vd, 

 vas deferens; vd', its opening on 13 th appendage. 



FIG. 402. Nervous system of crab, Carcinus (from Gegenbaur). a, antennal 

 nerves; c, cesophageal commissures; gi, fused ventral chain perforated for sternal 

 artery; gs, brain; o, optic nerve. 



(Pencils) the series is rendered more complete by the appearance of a nauplius 

 and a metanauplius with many appendages, before the zoeal stage. In the 

 crayfish and many land crabs the metamorphosis has been lost, but the lobster 

 leaves the egg in the Mysis stage. Differences may occur even in the same 

 species; thus in the European Palcemonetes rarians the embryo, in the sea, leaves 

 the egg as a zoea; in fresh water in the Mysis-stage. 



Sub Order I. MACRURA. Abdomen large; antennae long; ventral nerve 

 chain elongate; no megalops in development. CARIDEA. Body compressed; 

 no sutures on carapace; feet weak, external maxillipeds pediform; a large scale 

 on second antennae. PENEID/E, weak exopodites. Peneus* Sicyonia* PAL- 

 #:MONID/E, mandibles bifid at tip. Palcem-.m, Alpheus* Hippolyte* Pandalus* 

 CRANGONID.E, mandible simple. Crangon* Sabinea* ASTACOIDEA. Car- 

 apace crossed by a transverse groove. ASTACID^E have well-developed chelae. 

 Cambants* includes the crayfish of the eastern states; those of the Pacific coast 

 and Europe belong to Astacus* Ho mams,* lobsters. PALINTJRID.E (Loricata), 

 no chelae, body with heavy armor; larvae leaf -like (fig. 403). Palinitrus* spiny 

 lobster. PAGURIDEA, hermit crabs; abdomen reduced, soft-skinned, and 

 hidden in a snail shell which the animal carries about, which has resulted in a 

 spiral abdomen. Some hermits carry sea anemones or hydroids on their shell, 



