THORAX. 65 



examined were simply occupied by delicate corium, and no 

 doubt must aid in exposing a greater surface of corium to 

 the circumambient water. 



In my former volume on the Lepadidae, I have described 

 the ovigerous frcena occurring on the two "sides of the sack, 

 to which the ovigerous lamella are attached by a peculiar 

 glandular secretion : in the Balauidae there are no ovigerous 

 fraena, but the branchiae just described are identical with 

 the fraena in essential structure and in position ; differing 

 only in being placed a little nearer to the carinal end of the 

 sack, and in being generally (but not always) larger and 

 more plicated : seeing this, and that in Alcippe lamp as, and 

 in some species of Pollicipes, — the genus which comes 

 nearest to the Balanidae, — the ovigerous fraena are large and 

 are destitute of glands, and have therefore lost their normal 

 function of supporting the ovigerous lamellae, I can hardly 

 doubt that the branchiae in the Balanidce are the ovigerous 

 frama of the Lepadida? in a modified condition ; a trans- 

 formation of function not greater than that of the swim- 

 ming bladder of a fish into the lungs of the higher 

 Vertebrata.* 



Parts of the body included within the shell or carapace. 



These parts (PI. 25, fig. 1) consist of the prominent 

 mouth, and of the thorax (c'), with its largely developed 

 portion, called the prosoma (c), and with its appendages. 

 The abdomen is quite rudimentary, being represented 

 merely by a small portion of membrane surrounding the 

 anus, and sometimes inserted like a wedge between the 

 inwardly inflected posterior thoracic segments ; in only 

 two genera (Catophragmus and Pachylasma), its nature is 

 rendered somewhat plainer by supporting caudal appen- 

 dages. The probosciformed penis lies folded under the 

 thorax ; and I believe (from what is seen in the anomalous 

 genus Proteolepas), that it normally arises from the ventral 



* There is, I conceive, no foundation for the belief of some authors that the 

 branchiae of the Balanida? are in any way connected with the ovaria. 



5 



