CLASSIFICATION OF THE GASTEKOPODA. 



533 



Fig. 268. 



or general covering of the animal, conducts the respired fluid. The 

 branchiae themselves, as the name of the order indicates, are pectinated, 

 and form a single, double, or triple series of gills suspended from the 

 roof of the branchial chamber, answering the same intention as the 

 pulmonary network of the Snail, but deriving their supply of air from 

 the water in which they are perpetually immersed. In the figure 

 referred to, representing a species of Pterocera, the position of the 

 branchial chamber is seen through the shell and mantle, which the 

 reader must suppose to be transparent ; and the branchial organ ()> 

 in this case single, is likewise represented in situ, suspended from the 

 roof of the cavity that contains it. 



(1410.) In fig. 275 the roof of 

 the respiratory cavity (00} has 

 been reflected, and the three rows 

 of branchial fringes (n) suspended 

 therefrom are well seen. 



(1411.) A sixth order of Gas- 

 teropods has been formed by 

 Cuvier under the name of TUBTJLI- 

 BRANCHTATA, remarkable from the 

 shape of their shells, which are 

 long and irregular tubes, usually 

 fixed to foreign bodies, but still 

 they have the earliest-formed por- 

 tion twisted into a few spiral 

 curves. To this order belongs 

 Vermetus (fig. 268), the shells of 

 which, agglomerated into masses, 

 might be taken for those of cer- 

 tain Serpulce. As locomotion is 

 here out of the question, owing to the immoveable condition of the 

 habitations of such genera, the foot would seem at first to be altogether 

 deficient, but upon close inspection it is found to be converted into a 

 fleshy organ that bends forward and projects beyond the head, where its 

 extremity expands into a disk furnished with a small operculum ; so 

 that, when the animal retires into its abode, a lid is formed adapted to 

 close the aperture, and thus prevent intrusion and annoyance from 

 without. Nevertheless even in these the branchiae are pectiniform, 

 forming a single row attached to the roof of a branchial chamber. 



(1412.) The SCUTIBEAXCHIATA likewise have pectinated gills disposed 

 in a special cavity ; but their shells are very wide, and scarcely ever 

 turbinated a circumstance which, combined with other features of 

 their economy, renders it convenient to consider them as forming an 

 order by themselves. 



(1413.) An eighth division of this extensive class takes the name of 



Vermetus. 



