108 TUBULIBRANCHES. CYCLOBRANCHES. 



and the wide-cheeked Strombus, all of them being possessed 

 by snails essentially alike in their anatomical structure and 

 their leading habits. 



7. Tubulibranches. — Similar in their organs of respira- 

 tion to the Pectinibranches, from which Cuvier detaches 

 them because of their being asexual or hermaphroditical, — a 

 structure necessitated by their fixed condition ; for, unlike 

 all other Gasteropods, the shell of the Tubulibranches is 

 immovably fixed to foreign bodies. This fixidity is accom- 

 panied with a considerable modification in the foot of the 

 animal, which, its occupation gone, is reduced to a rudi- 

 mentary state ; and in the shell, which is spiral only at its 

 apex, and ends in a long fiexuous or straight tube, bearing 

 a strong resemblance to the serpulous shell of some worms, 

 with which, indeed, the Vermetus and Siliquaria, the prin- 

 cipal genera of this order, have been often classified. 



8. Scutibranches. — This comprises a small number of 

 Gasteropods likewise very similar to the Pectinibranches in 

 the form and position of the branchiae as well as in the general 

 form of the body ; but, like the last, they are hermaphrodites, 

 and, unlike them, have the power of walking to and fro. 

 Their shells are widely open, ear-shaped or patelloid, non- 

 operculate, so that they cover the animal like a buckler ol- 

 dish rather than contain it. In their interior anatomy they 

 make an approximation to the bivalvular mollusca, and their 

 shell has a certain resemblance to a single valve of that class. 

 The Halyotis, or ear -shell, the Fissurella, and Emarginula, 

 separated from the Patella of Linnaeus, are the principal 

 examples. 



9. Cyclobranches. — Another small hermaphroditical 

 family distinguished by their branchiae, which, in place of 

 being situated within a peculiar cavity, form a filamentous 



ribbon between the margin of the cloak and 

 Fig. 16. foot, nearly encircling the body. The genera 



aTg-ii are Patella and Chiton (Fig. 16), exceedingly 



dissimilar in their external appearance,- — the 

 former being covered with a simple conical 

 shell, the latter with a series of testaceous 

 plates arranged along and across the back. 



The fourth class of mollusca — the Acephales — are all 

 aquatic animals and very numerous. The first order in it, 

 named A. Testaces, have the respiratory organs in the form 

 of four broad leaves, a pair on each side of the body, which 

 again is always contained within a bivalvular shell, that, in 

 a few instances, has some additional pieces affixed over the 

 hinge. The cockle, the mussel, and the oyster, are members 



