THE CRUSTACEA. 125 



adult condition, but leave the egg as actively locomotive larvae, 

 provided with a discoid carapace and three pair of appendages, 

 which represent the antennules, antennae, and mandibles of the 

 higher Crustacea. After a time, these larvae moult, the cara- 

 pace becomes bivalved, the antennules are converted into 

 organs of prehension, and several pair of paddle-like thoracic 

 limbs appear. The further state of the larvae differs in the two 

 principal subdivisions of this order. 



In the Bhizocephala, which are parasitic upon other Crus- 

 tacea, the anterior region of the body gives out filaments, which 

 grow like roots into the body of the animal which bears the 

 parasite. 



In the Cirripedia the larvae first adhere to foreign bodies 

 by the suckers developed upon the prehensile antennae, and 

 then glue themselves in this position by the secretion of a 

 " cement gland." The anterior part of the body next becomes 

 enlarged either in breadth or length, or both, into a peduncle ; 

 and, usually, definite calcifications occur in the carapace. 

 The thoracic limbs of the larva are commonly preserved as 

 six pair of cirrated appendages, and the animals are very 

 generally hermaphrodite. Sometimes, however, they are 

 dioecious ; and some of the hermaphrodite forms have " supple- 

 mental males." 



The " Acorn Shells " and Barnacles are common examples 

 of Cirripedia. 



5. In the STOMAPODA the axis of the ophthalmic and an- 

 tennulary somites is parallel with that of the body generally, 

 and these two somites are ordinarily movable on one another. 

 There is a carapace developed from the head. The eyes are 

 on movable stalks. The heart, elongated and many-chambered, 

 extends through the abdomen. The branchiae are attached 

 to the abdominal appendages. 



To this order the Squillse belong. 



6. The EDBIOPHTHALMIA have sessile eyes. At fewest, five 

 of the posterior thoracic somites are free and movable, and the 

 anterior pair of thoracic appendages are converted into foot- 

 jaws. There is no carapace. The heart is many-chambered, 

 and the branchiae may be thoracic or abdominal. 



