20 



Guide to the M oilmen. 



been obtained from depths of over 2,000 fathoms. About 600 

 living species are known, including more than a dozen from 

 British coasts. 



These animals, like woodlice, have the power of rolling them- 

 selves into a ball. In some of them the eggs are incubated in the 

 furrow between the gill and the foot ; in Callistochiton viviparus 

 development is direct, and takes place in the oviduct. 



The Aplacophora (Case 3) are worm -like and devoid of shells ; 

 the mantle usually covers the body completely, a median ventral 

 furrow remaining uncovered in Neomenia and its nearest allies. 

 The foot is very much reduced in size or is absent altogether. 

 The gills are contained in a special cavity at the posterior end of 

 the body. These animals live on oozy bottoms, sometimes at very 

 great depths. Neomenia and allied genera are usually found upon 

 corals and Hydroids. A few species occur in British Seas. 



Fig. 10. 



Proneomenia gerlachcri, left side view. 



b. mouth ; cl. cloaca ; si. foot groove ; t. head. 



From Lankester's Treatise on Zoology (by permission of Messrs. A. & C. Black). 



Class II. GASTROPODA. 



(Cases 3 to 136.) 



This class contains some of the most familiar molluscs, such as 

 the Limpet, the Whelk, the Snail, and the Slug. Most Gastropods 

 have a spirally coiled shell and a well-differentiated head region, 

 and their internal organs are asymmetrical. During the develop- 

 ment of the Gastropoda the original symmetry of the larva is 

 affected by two processes known as ' ventral flexure ' and 

 ' torsion '. As a result of these the anus, which was originally 

 posterior, becomes anterior and ultimately reaches a position 

 above the mouth, the visceral commissure of the nervous system 

 is twisted into a figure of eight, and certain organs of the original 

 left side tend to atrophy and may disappear. There are many 

 Gastropods in which the visceral commissure is untwisted, and 

 some of these are symmetrical and have the anus in its original 

 posterior position ; but they have only one auricle of the heart, 

 and one kidney, and it seems clear that their apparently primitive 



