HIGH SCHOOL ZOOLOGY. 



235 



rieties of spirals are to be met with in t]ie Top-shells, ( Turbo and Tro- 

 chus), Olives, Cone-shells, Cowries (Cypriea), &c., &c. Some few Proso- 

 branchiates, such as Helicina, Valvata, Paludina, are met with in fresh 

 water (Figs. 159, 6 and 7). 



22. In the Heteropoda we 

 have a series of Gastropods 

 adapted for a pelagic life, the 

 foot being compressed into a fin, 

 and the visceral mass and its 

 protecting shell much reduced 

 in size, so as not to interfere 

 with the transparency of the 

 creature (Fig. 160); and in the 

 OpisthobrancMata, a series of 

 marine forms, in which the 

 shell is small or absent, but the 

 gills generally project free from 

 the surface of the body, and are 

 situated behind the heart (Fig. 

 161). 



Fi?. 160.— Outline of 

 a Heteropod (Atlanta). 

 The foot is divided into 

 three regions : pro- meso-, 

 and nieta-podiuui. On the 

 niesoijodiuni is a suclier, 

 on tlie metapodium, an 

 operculum. 



Fi-. 161. 



A naked 

 Opistholsran- 

 chiate (Doris), 

 A rosette of gills 

 surrounds the 

 anus. 



23. The Gastropods are the only Class of Cephalophorous Mol- 

 luscs which are represented inland ; the others are exclusively 

 marine, and embrace compai'atively few living forms. They are 

 contained in three Classes, the Scaphopoda or Tooth-shells, with 

 burrowing foot and numei'ous slender tentacles (Fig. 162 b), the 

 Pteropoda, pelagic forms with or without a shell, but with the 

 foot converted into two wing-like fins (Fig. 162 c), and the 

 Cephalopoda> at present a small group in comparison with its 

 development in past geological times (Fig. 162 a\ To 

 this class belong the most highly-organized Mollusca — the 

 Cuttle-fishes (Sepia), Squids (Loligo), Octopus, itc; in all 

 of which the shell is reduced to an internal " cuttle-bone " 

 or "pen." Tlie foot is partly transformed into a circlet of 

 ten or eight " arms " surrounding the mouth and carrying for- 

 midable suckers, and partly into a " funnel," which permits the 

 water used in respii-ation to be forcibly ejected from the mantle- 



