192 Elementary Zoology. 



respiratory, and which contain branches of the liver, and are 

 armed externally with thread-cells like those of the Ccelentera. 



4. Pteropoda. — The Pteropods, allied to the Opistho- 

 branchs, are pelagic creatures of a transparent appearance. 

 The foot is somewhat reduced, but in compensation two 

 lateral processes of the foot — present also in some of the 

 Mollusca already considered — are largely developed, and form 

 the swimming organs ; these are known as the epipoda. The 

 Pteropods have complicated reproductive organs, and are 

 hermaphrodite. Though there is a pseudo-symmetry, the 

 nephridium and the auricle are single. Some have, and others 

 have not, a mantle-fold and a shell. 



5. Pulmonata. — This group of Molluscs comes nearest to 

 • the Opisthobranchs, and includes, not only the terrestrial snails 



and slugs, but also the fresh-water Flanorbis (Fig. 84), and the 

 pond-snail, Lymdcea (Fig. 83). The Pulmonates have no gills, 

 respiration being eifected by the very vascular walls of the 

 pulmonary chamber, formed by a fold of the mantle. The shell 

 is usually present, but may become atrophied, as in some slugs. 

 The slug Testacella, however, has a small shell. The generative 

 organs are complicated, and the animals are hermaphrodite. 



6. Scaphopoda. — To this group many give equal rank with 

 the larger divisions of the Glossophora. It is, however, more 

 closely allied to the Cephalophora than to any other of the 

 four main divisions allowed here. The Scaphopoda have a 

 characteristic shell, shaped like a truncated elephant's tusk ; it 

 is open at both ends, one being narrower than the other. This 

 shell has a cylindrical form on account of the fact that the 

 mantle which secretes it has fused along the ventral surface of 

 the body, thus forming a complete cylinder. Two nephridia 

 are present, and the generative gland is furnished with a duct 

 which opens into the right of these : the duct is, however, not 

 complicated. There is no heart present. 



4. Class CEPHALOPODA. 



The cuttle-fishes are bilaterally symmetrical animals with a 

 well-marked head and a radula. There is usually a shell 



