:5l> 



(luidt to the Mothisca. 



They exist in countless millions in some parts of the ocean, 

 discolouring the water for miles. In certain regions the bottom 

 of the ocean, in depths between 1,000 and 2,000 fathoms, is 

 covered by a thick ooze formed from their shells. 



The Sea-Hares, so called on account of a slight resemblance to 

 a crouching hare, are found in most parts of the world. At the 

 hinder pari of the back two flaps of the mantle partly conceal 

 a thin horny shell which serves as a protection to the gills and 

 vital organs beneath. When molested, these animals discharge 

 a large quantity of a purple fluid, discolouring the surrounding 

 water for a distance of more than a yard. 



The remarkable shell of Umbraculum is shaped very like 

 a Chinese umbrella. 



The Nudibranchs or Naked-gilled Molluscs comprise some of the 

 most beautiful and strange forms. They are unprovided with 



Fig. 23. 



Naked-gilled Mollusc, or Nudibranch (Doto coronata). 

 a. head ; b. foot ; c. dorsal papillae ; d. tentacle-sheath ; e. tentacle. 



shells except in the earliest stages of their existence, when they 

 have a minute coiled (and often nautiloid) shell, furnished 

 with an operculum, both of which are subsequently cast off. 

 Some of them bear on the back a series of appendages, often 

 fantastically shaped. Unfortunately the colours of these beauti- 

 ful creatures cannot be preserved after death, and therefore 

 a small series of glass models is exhibited, which will give some 

 idea of their great variety in form and colouring. They are found 

 in most parts of the world, chiefly in shallow water, but a few 

 species live upon floating seaweed in the open sea. Over a 

 hundred species exist on the British coast, the majority of which 

 are, however, very small. They are chiefly carnivorous, feeding 

 on other molluscs, Sea- Anemones, &c. 



Order 2. PULM0NATA. 



(Case* 97 to 135.) 



The Pulmonata are Euthyneura in which the mantle-cavity is 

 converted into a lung. There is no true gill, but in certain aquatic 



