CHAP. II. TESTACEOUS ANIMALS IN GENERAL. 33 



that Ave have the most perfect examples of the sedentary 

 Testacea. 



(27.) Although the head, where it exists, of the Tes- 

 tacea, is very different from that of vertebrated and 

 annulose animals, — inasmuch as it is not separated 

 from the body by a neck, — it is yet to be distinguished, 

 in many groups, by the presence of tentacula, or feelers, 

 and by two or four black dots, which are generally con- 

 sidered to be the organs of sight : in proportion as we 

 advance from the Planarice on one side, and from the 

 chitons on the other, towards the cuttlefish, we find 

 the head and the eyes gradually assuming that definite 

 shape and structure which are so characteristic of the 

 vertebrated animals. Some of the tritons (^Nudi- 

 hrancMa'), and all the bivalve shell-fish {Difhyra),ha.ve 

 no indication of eyes, nor have the latter any obvious 

 head ; but in the Gasteropoda, or univalves, both begin 

 to appear. AVe see this in the garden snail, and in all 

 the freshwater and marine animals of this tribe. In 

 the StrombidcE and the Volntidce, the eye is nearly as 

 perfect as that of the cuttlefish. From this group, 

 however, nature again recedes ; and in the Pteropoda, 

 although the general form is preserved, the head is once 

 more confounded with the body, and the eyes disap- 

 pear. 



(28.) On the habits, food, and geographic distribu- 

 tion of the Testacea we can say but little which is 

 applicable to all. The great majority are aquatic and 

 marine ; but two or three extensive families are found 

 only in fresh water. The land shells are exceed- 

 ingly abundant in species, and the gastropod worms 

 are parasitic in or upon other animals. It may be here 

 remarked, that all the bivalves {Dithyra) are aquatic, 

 but that the univalves inhabit the diflTerent situations 

 just enumerated. The food of all these creatures varies 

 according to their own particular races. A large num- 

 ber (forming the phytophagous gastropods) feed almost 

 only upon living vegetables, either terrestrial or aquatic, 

 — as the snail of our gardens, and the periwinkle of our 



D 



