xi PHYLUM ARTHROPODA 585 



in which it resembles an adult Mysis, having exopodites to all 

 the thoracic limbs. 



In the Crabs the nauplius stage is passed through in the egg, 

 and the young is hatched in the form of a peculiarly modified 

 zosea (Fig. 482, A), with an immense cephalothorax produced into 

 spines, large stalked eyes, and a slender abdomen. This passes 

 by successive moults into the megalopa-stage (B), which resembles 

 an adult Macruran, having an extended abdomen with well- 

 developed pleopods. The megalopa passes by successive moults 

 into the adult form. 



In the Lobster (Homarus) both nauplius and zosea-stages are 

 passed through in the egg, and the embryo is hatched in the mysis- 

 stage with exopodites to all the thoracic limbs. In the Rock- 

 lobster (Palinurus), and its allies, the newly hatched young is a 

 strangely modified mysis-form called a Glass-Crab or PJiyllo- 

 soma : it has broad, depressed cephalic and thoracic shields of 

 glassy transparency : the abdomen is very small and the legs 

 extremely long and biramous. Lastly, in the Fresh-water Cray- 

 fish the young resemble the adult in all but proportions and certain 

 unimportant details of structure. Thus in the series of Decapoda 

 we get a gradual abbreviation in development, stages which are 

 free larval forms in the lower types being hurried through before 

 hatching in the higher. 



The larvae of Stomatopoda are grotesque little creatures with a 

 very large spiny carapace. In Amphipoda there is no free larval 

 form, but in Isopoda the young leave the egg in the form of a 

 curious maggot-like modification of the nauphus, which remains 

 in the brood-pouch until it has attained the adult form. 



Ethology. The Crustacea are remarkable for their very perfect 

 adaptation to the most various conditions of life : they occur in 

 fresh-water, in the sea, in brine-pools, in subterranean caves, and 

 on land : of the marine forms some are littoral, some pelagic, some 

 abyssal, descending to over 3,000 fathoms. One species of Copepod, 

 Pontellina meditermnea, may almost be considered as aerial : it 

 is described as taking long flying leaps out of the water, after the 

 manner of a Flying-fish. Some, like Lobsters, Crayfishes, &c., are 

 solitary ; others, like Shrimps, are gregarious, occurring in immense 

 shoals. Most of them either prey on living animals or devour 

 carrion, but, as we have seen, the Barnacles are fixed, and feed on 

 minute particles after the fashion of many of the lower animals, 

 and the members of more than one order are parasites remark- 

 able for their deviation from the typical structure of the class 

 and their adaptation to their peculiar mode of life. In size they 

 present almost every gradation from miscroscopic Water-fleas 

 to Crabs two feet across the carapace, or four feet from tip to 

 tip of the legs. 



