278 CLASSIFICATION OF ANIMALS 



The vent is usually on the right side anterior to the visceral mass. The 

 heart consists of a ventricle and one or two auricles. It lies in a small 

 body cavity called a pericardial chamber. The kidney communicates 

 with the pericardial chamber by a nephridial funnel and opens into the 

 mantle chamber through a duct — the ureter. 



597. The nervous system consists of a pair of cerebral ganglia, pleural 

 ganglia and pedal ganglia which are all closely connected into a nerve 

 collar. There are also parietal and buccal ganglia. From these ganglia 

 nerves are supplied to the sense organs and muscles of the head, to the 

 mouth, to the foot, to the gills, olfactory organs (osphradia) and a part 

 of the mouth, and to the buccal mass and intestine respectively. The 

 sense organs usually present are tentacles, eyes, a statocyst which is 

 usually close by the pedal ganglion though it is innervated from the brain, 

 and chemical sense organs, called osphradia, located on or near the gills. 



598. Some of the Gastropods are hermaphroditic, in others, the sexes 

 are separate. The reproductive system is frequently very complicated 

 for besides the gonads and their ducts which may be variously modified, 

 there may be two, three or more kinds of glands and other accessory 

 reproductive organs. Development is either direct or by metamorphosis. 

 The embryo is at first symmetrical. A larva known as a veliger occurs 

 in many forms. 



599. The Gastropods are typically aquatic but there are many forms 

 in which the mantle chamber serves as a lung, no gills being developed. 

 This is the case with many fresh-water forms and a large number of 

 forms which are purely terrestrial. Many Gastropods are vegetable 

 feeders. Others are carnivorous, some have the power of boring through 

 the shells of other molluscs by means of an acid secretion, and thus killing 

 their prey. 



600. The numerous families of Gastropods are classified as follows: 



Legion I. Streptoneura 



Sub-order i. Aspidobranchia 



Sub-order 2. Ctenobranchia 



Sub-order 3. Heteropoda 

 Legion II. Euthyneura 



Sub-order i. Opisthobranchia 



Sub-order 2. Pulmonata. 



601. The Streptoneura have the visceral nerves crossed like a figure 8 

 because of the twisting of the visceral mass. For the same reason the 

 gills lie in front of the heart. The sexes are generally distinct. The 



