164 



A COURSE ON ZOOLOGY. 



FIG. 94. 



form of a U, that is, the two extreme orifices are quite 

 close to each other. Besides, the relations of the intes- 

 tine with the heart are very intimate, and often the 

 heart straddles the intestine. The liver is very large. 



The heart is arterial, that is, it is traversed by red 

 blood, and there are arterial vessels and venous vessels. 

 However, there are no capillaries, these being replaced 

 by lacunae between the organs and in the muscles. 



The respiratory organs differ both in their structure, 

 their form, and their arrangement. We must note in a 

 general manner that they are placed under the mantle, 

 which forms a respiratory chamber designed to contain 

 either air or water. Ordinarily respiration is aquatic, 

 and is effected by gills usually located 

 on the exterior ; however, in certain mol- 

 lusks, naked snails for example, it is aerial, 

 and has for its seat an interior cavity 

 that has been compared to the lungs. It 

 is a curious fact that among the pul- 

 monary mollusks several species are 

 aquatic. The latter, like the correspond- 

 ing members of the insect class, are 

 obliged to seek frequently the surface 

 of the water for a supply of air. 



The nervous system always presents 

 an oesophageal collar, joining the cerebral 

 ganglions with those situated in the foot, 

 in the base of the gills, or in the mouth ; 

 but the latter ganglia are not united in a 

 chain or linear series as in the anarthropods. 

 All mollusks are oviparous. 



We cannot go into detail on the classification of mol- 

 lusks. The division includes six orders, of which only 



NERVOUS SYSTEM 

 OF A MOLLUSK. 

 1, oesophageal 

 chain ; 2 and 3, 

 ganglia. 



