44 RESPIRATORY CAVITY OF PECTINIBRANCHIATA. 



and many of those in which the shell is simply conical. 

 most remarkable feature of almost all these animals is the ar- 

 rangement of the branchial apparatus; but this character is not 

 constant ; for in two genera (cyclostoma and helicina), which, in 

 other respects, are too nearly allied to the ordinary pectinibran- 

 chiata to be separated from them, there are no branchiae, and 

 their respiration, which is aerial as in pulmonea, is carried on 

 by means of a vascular net-work that lines the bottom of the 

 respiratory cavity. This cavity occupies the last whorl of the 

 shell and opens externally by a great slit, situate betwixt the body 

 and the edge of the mantle; in most instances it lodges branchiae 

 (Jig. 32, fe), which are composed of small leaves or fringes ar 

 ranged parallel and attached to its superior parietes upon one, two, 

 or three lines, according to the genus. All these mollusks have two 

 tentacles (to) and two eyes, sometimes borne on particular pedun- 

 cles ; the mouth is in the form of a tube or trunk (f), and encloses 

 a tongue armed with small hooks ; the males have, on the right 

 side of the neck, .an appendage, sometimes very thick, which 

 cannot, in general, be drawn into the body of the animal, but is 

 folded into the branchial cavity ; the rectum and the oviduct (ov) 

 are also found on the right side of this cavity, and near them we 

 remark a particular organ, enclosing a very viscid humour, de- 

 signed to form a common envelope around the eggs. 



27. This order is divided into three families. In the two 

 first (that of the Trochoides and of the Capuloides), there is no 

 syphon by the aid of which the animal can breathe without 

 leaving its shell (Jig. 33), while in the third family (that of the 

 Buccinoides) there is a respiratory tube, formed by a prolonga- 

 tion of the edge of the pulmonary cavity of the left side, which 

 passes through a corresponding canal or notch in the shell 

 (fig. 34). 



Fig. 33. TURRITELLA. Fig. 34. CASSIS. 



27. How is the order of pectinibranch ga'steropods divided? How doss 

 the family of Buccinoides differ from the other two families the Trochu uies 

 ivnd Capuloides ? 



