1 08 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 flexuous 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 or 

 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 



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 



