274 



CRUSTACEA. 



and zoologists, before J. Vaughan Thompson's researches (1829), were 

 satisfied with calling Cirripedes divergent Molluscs, we now know clearly 

 that they are somewhat degene- 

 rate Crustaceans. We do not 

 know, however, by what con- 

 stitutional vice, by what fatigue 

 after the exertions of adoles- 

 cence, they are forced to settle 

 down to sedentary life. 



The food consists of small 

 animals, which are swept to 

 the mouth by the waving of the 

 curled legs. Growth is some- 

 what rapid, but the usual skin 

 casting is much restricted ex- 

 cept in one genus. Neither the 

 valves, nor the uniting mem- 

 branes, nor the envelope of the 

 stalk, are moulted, though dis- 

 integrated portions may be re- 

 moved in flakes and renewed 

 by fresh formations. In the 

 allied genus ScalpeUiun, some 

 are like Lepas, hermaphrodites, 

 without complementary males 

 (Sc. balanoides) ; others are 

 hermaphrodite, with comple- 

 mentary males {Sc. villosuiit), 

 &c. ; and others are unisexual, 

 but the males are minute and 

 parasitic (Sc. regiunt). 



Balanus^ the acorn shell, en- 

 crusts the rocks in great num- 

 bers between high and low 

 water marks. It may be de- 

 scribed, in Huxley's graphic 

 words, as a crustacean fixed by 

 its head, and kicking the food 

 into its mouth with its legs. 

 The body is surrounded, as in 

 Lepas, hy a fold of skin, which 

 forms a rampart of six or more 

 calcareous plates, and a four- 

 fold lid, consisting of two scu/a 

 and two ferga. When covered 

 by the tide, the animal pro- 

 trudes and retracts between the 

 valves of the shell six pairs of 

 curl-like thoracic legs. The 

 structure of the acorn shell is 

 in the main like that of the barnacle, but there is no stalk. 



Fig. 90. — Development of Sacculina. 



(After Dei..-\ge.) 



(Not drawn to scale.) 



A. Free swimming Nauplius, with three 

 pairs of .-ippendages ; H. Pupa stage ; C. 

 Adult protruding from the tail of a crab. 



