406 



GASTEROPODA. 



reflected, and the three rows of branchial fringes (n) suspended 

 therefrom are well seen. 



A sixth order of Gasteropods has been formed by Cuvier, 

 under the name of TUBULIBRANCHIATA, 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 

 portion twisted into a few spiral curves. To this order belongs 

 Vermetus (Jig. 187), the shells of which, agglomerated into masses, 

 might be taken for pi gf 137, 



those of certain Serpu- 

 I<E. As locomotion is 

 here out of the ques- 

 tion, owing to the im- 

 moveable condition of 

 the habitations of such 

 genera, the foot would 

 seem at first to be al- 

 together deficient, but 

 upon close inspection it 

 is found to be convert- 

 ed into a fleshy organ 

 that bends forward and 

 projects beyond the 

 head, where its extre- 

 mity expands into a disc 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 bran- 

 chiae are pectiniform, forming a single row attached to the roof of 

 a branchial chamber. 



The SCUTIBRANCHIATA likewise have pectinated gills dis- 

 posed 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. 



An eighth division of this extensive class takes the name of 

 CYCLOBRANCHIATA, because the branchiae form a fringe around 

 the body of the animal, between the edge of the body and the 

 foot (fig. 194, c ; fig. 197, a). 



Lastly, a distinct order has been established to embrace certain 

 families in which the foot is so much compressed as to constitute 



