260 THE ANATOMY OF INVERTEBRATED ANIMALS. 



duncle, which is produced by the gradual elongation of the 

 " beak " of the pupa. 



With the assumption of its perfect form, the Cirripede 

 ceases to moult its carapace, ecdysis being hereafter confined 

 to the inner lining of the sac, and to the integument of the 

 contained body. 



Such is the structure and development of a typical pedun- 

 culate Cirripede. In other genera, such as Pollicipes, calca- 

 reous plates are developed on the peduncle, foreshadowing 

 the compartments of the sessile forms. The latter, of which 

 JBalanus may be regarded as the type, differ in structure from 

 Lepas in no very essential particular. The peduncle, very 

 short and broad, instead of slender and elongated, is incased 

 by its compartments, and is sometimes fixed by a shelly basis. 

 The arrangement of the layers of cement is often extremely 

 complicated; the scuta and terga are articulated together; 

 the frasna are much larger organs, and posssibly subserve the 

 respiratory function ; the thoracic ganglia are concentrated 

 into a single mass ; and the cementing apparatus is much 

 more complicated. 



The pedunculate and sessile Cirripedia, taken together, 

 constitute by far the largest of the three great groups which 

 Mr. Darwin recognizes ; namely, the Thoracica, characterized 

 by having limbs attached to the thoracic somites, while the 

 abdomen is rudimentary. 



The second group, the Abdominalia, contains only one 

 genus, Cryptojjhialus (Fig. 69, 5, 6), which has no thoracic 

 limbs, but is provided with three pairs of abdominal append- 

 ages. The larva is very imperfect in its first and second 

 changes, which are undergone within the sac of the parent. 



The third group, Apoda, likewise contains only one 

 genus, the remarkable Proteolepas (Fig. 69, 7), which is 

 devoid of either thoracic or abdominal limbs ; it has a vermi- 

 form body, and a rudimentary peduncle, represented by two 

 threads terminated by the characteristic antenniform organs. 



In the great majority of the Cirripedia the sexual appa- 

 ratus is disposed as in Lepas, but Cryptophicdus and Alcippe 

 are unisexual, the male differing very widely in form and size 

 from the female (Fig. 69, 3, 6). 



The JBalanidm, or sessile Cirripedes, all present the nor- 

 mal sexual relations ; but the other division of the Thoracica, 

 the Pepadldce, contains two genera, Ibla and Scalpellum, 

 which not only possess species having the sexes in distinct 

 individuals, but others presenting the unique combination of 



