CLASSIFICATION OF CRUSTACEA. 247 



longer spined tail, and stronger legs. Then it passes into a Cypris- 

 stage, with two side eyes, six pairs of swimnaing legs, a bivalve shell, 

 and other organs. As it exerts itself much but does not feed, it is not 

 unnatural that it should sink down in over-fatigue. It iixes itself by its 

 head and antennse, and is glued by the secretion of the cement gland. 

 Some of the structures, e.g. , the side eyes and the bivalve shell, are lost ; 

 new structures appear, e.g., the characteristic Cirriped legs and the 

 shell. Throughout this period, which Darwin called the " jiupa-stage,' 

 there is external quiescence, and the young creature continues to fast. 

 The skin of the pupa moults off; the adult structures and habits are 

 gradually assumed. At frequent periods of continued growth, the whole 

 lining of the shell and the skin of the legs are shed. In spring these 

 glassy cast coats are exceedingly common in the sea. Acorn-shells feed 

 on small marine animals. They fix themselves not to rocks only, but 

 also to shells, floating objects, and even to whales and other animals. 



Alcippe and Crypiophialus (with only three or four pairs of feet) live in 

 the shells of other Cirripedes or of Molluscs ; Froteolepas is parasitic in 

 the mantle of other Cirripedes, and like a grub. 



The abdomen of a crab or of a hermit-crab often shelters some 

 species of Sacculina or of Peltogaster, the most degenerate of all parasites. 

 The creature is little more than a sac of reproductive organs, and yet it 

 started in life as a Nauplius, and became a Cyprid larva like many another 

 Crustacean. The body is a rounded sac, corresponding to the head 

 region of the tar\'a. There is no food-canal, nor trace of segmentation 

 or appendages. The sac is fixed by absorptive " roots," which have 

 insinuated themselves into the viscera of the crab. The sac-like body 

 contains a brood-cavity and hermaphrodite reproductive organs, but 

 sometimes there are associated pigmy males. 



Second Sub-Class. Malacostraca. 



These are higher Crustaceans in which the body consists 

 of three regions with a constant number of segments, five 

 to the head, eight to the thorax, and six to the abdomen 

 (except in forms Uke Nebalia, which have seven). The ter- 

 minal piece or telson of the abdomen is regarded by some 

 zoologists as a distinct segment. Apart from this telson, 

 and also the segment next to it in Nebalia, all the segments 

 bear paired appendages. More or less of the thorax is fused 

 to the head region, and the anterior thoracic limbs are usually 

 auxiliary to mastication. Two compound lateral eyes and 

 a gastric mill are always present. The female genital aper- 

 tures are on the third last pair of thoracic legs, the male aper- 

 tures on the last pair. Very few are hatched in the Nauplius 

 stage, many however at the Zosea level, while others have no 

 metamorphosis at all. 



