MOLLUSCA GASTEROPODA. 



105 



(Brancliw-G aster opoda) perform the respiratory function 

 in three different ways 



1. Some of the Ileteropoda have no special breathing 

 organs. The walls of the mantle-cavity are supplied 

 with blood-vessels, and the blood in passing through 

 these is purified by the air contained in the water. 



2. The sea-slugs, &c. have external breathing organs, 

 consisting of tuft-like expansions of the integument 

 placed on the back and sides of the animal. These are 

 called Nudibranchiata, (Lat. nudus, naked ; Gr. branchia, 

 gills), and may be found on sea-weeds or under stones on 

 the sea-shore. 



FIG. 44. CARIXARIA AND ITS SHELL (Ileteropoda.) 



3. In the majority of cases, the gills are plume-like 

 organs, contained in a chamber, formed by a fold of the 

 mantle. In many members of this group, the water is 

 conveyed to the gill chamber through a siphon, formed 

 by a prolongation of the mantle fold. This water passes 

 off through an aperture which is often furnished with 

 another siphon. 



147. The Nervous System consists of three pairs of 

 ganglia the cerebral, pedal, and parieto-splanchnic. 

 These are united by nervous threads. There are two 

 eyes, often placed at the summit of a pair of stalks. In 

 the land snails, these stalks are tubular, and contain 

 muscular threads, which, by their contractions, pull the 

 eyes into the interior of the tubes, when the animal is 

 apprehensive of danger to these important organs. There 

 are two auditory vesicles, filled with fluid, and contain- 

 ing otoliths (Gr. ous, an ear ; lithos, a stone), which are 

 in a constant state of vibration. These organs are placed 

 at the bases of the tentacles. 



