THORAX. 67 



ever, the whole membrane of the five thoracic segments is 

 very thin, excepting small wedge-shaped portions along the 

 medio-dorsal line. The infolded articulations between the 

 segments supporting the three anterior pairs of cirri (at 

 least in the Balaninse), are much wider than those between 

 the three posterior segments ; the former segments, with their 

 cirri, being consequently capable of being moved further apart 

 from each other. Could there have been any doubt as to 

 the distinctness and reality of the five thoracic segments, it 

 would have been set aside by the arrangement of the 

 muscles attached to them, as will presently be described. 

 I need only add, that in many genera there are shield-like 

 swellings at the exterior bases of the pedicels of the posterior 

 cirri, which I for some time thought were the epimeral ele- 

 ments of the thoracic segments ; but I now believe them to be 

 parts of the pedicels of the cirri. The basi-exterior margin, 

 moreover, of the pedicel of the third pair of cirri, in many 

 species of the Balaninse (PL 25, fig. 1), is produced as a plate, 

 thickly fringed with fine hairs, half across the dorsal surface 

 of the thorax ; serving, apparently, as a brush to clean the 

 sack, or to prevent the ingress of any intruding substance. 

 The soft, rounded, bag-like portion of the body, which I 

 have called the prosoma, is usually separated by a notch 

 from the five posterior thoracic segments ; at its upper end 

 it may be said to carry the mouth and first pair of cirri. 

 The prosoma includes the main part of the stomach and the 

 broad ends of the vesicular seminales. It is always clothed 

 by very thin membrane, which in Chthamalus dentatus, is 

 hairy. In Tnbicinella and Xenobalanus, the prosoma is 

 much elongated, being produced far down the deep sack. 

 That the prosoma is mainly formed by a great development 

 of that segment (homologically the second thoracic segment) 

 which carries the first pair of cirri, is certain, and I should 

 not have hesitated to have said that it was exclusively so 

 formed, had not the first thoracic segment in the anoma- 

 lous genus Cryptophialus been developed as a distinct and 

 free segment, not attached to the carapace; showing that 

 possibly in other Cirripedes, the dorsal half of this first tho- 

 racic segment may be concerned in the formation of the free 

 prosoma. 



