SNAILS AND SLUGS 



325 



side of the body and almost entirely covered over. A large 

 gill is present, protected in a mantle-cavity which opens on the 

 right-hand side (see fig. 188). The external opening of the 

 single kidney is at the root of the gill, and the intestine terminates 

 still further back outside the rriantle-cavity altogether. The Sea- 

 Hare was at one time pointed to as a good example of a form 

 in which the twisting process had begun, carrying the mantle- 

 cavity with its organs on to the 

 right side. If this were so, how- 

 ever, we should expect to find two 

 auricles to the heart, two gills, 

 and two kidneys, which is not 

 the case. A more adequate 

 explanation is that the Sea- Hare 

 is descended from forms in which 

 coiled visceral hump and shell 

 were present, and which had lost 

 an auricle, a gill, and a kidney; 

 forms, in fact, resembling such a 

 species as the Periwinkle in struc- 

 ture (see p. 318). We must sup- 

 pose that in these the visceral 

 hump gradually flattened out and 

 the shell gradually became re- 

 duced, while at the same time a 

 certain amount of untwisting took 

 place, bringing back the mantle- 

 cavity to the right-hand side of 

 the body. In this way a secon- 

 dary or spurious simplicity has been acquired. The non-twisted 

 nerve-loop (fig. 188) is characteristic of Euthyneura generally. 



Eight out of the twenty families embraced by the Tecti- 

 branchs are collectively known as the Wing-footed Snails or 

 Pteropods (Gk. pteron, a wing; pous, a foot), formerly regarded 

 as a distinct class of the Mollusca. They are small pelagic 

 creatures, vast shoals of which are to be found swimming in 

 the open sea. Some of them possess a mantle-cavity and a 

 transparent shell either spiral or conical in shape. In these the 

 foot is transformed into a pair of fin-like structures. Others 

 again have no mantle and shell, and though they possess fins, 



Fig. 188.— Diagram of aTectibranch Snail, seen 

 from above 



I, Mouth; 2, nerve-ring with ganglia; 3, one of the 

 two ganglia on the untwisted nerve-loop; 4, gill, just in 

 front of which is seen the osphradium; 5, opening of 

 intestine; 6, heart in pericardium; 10, loa, right epipo- 

 dium; 10^, left epipodium folded over back. 



