204 ZOOLOGY 



Pterotrachea the visceral hump is still more reduced, and is 

 devoid of a shell. 



The sense organs and nervous system are unusually well 

 developed in this subdivision, the otocysts being usually 

 closely attached to the cerebral ganglia. The lingual ribbon 

 has moveable lateral teeth, which divaricate when the tongue 



Fig. 122. — Diagrams of a series of 

 Molluscs to show the form of 

 the foot and its regions and the 

 relations of the visceral hump 

 to the antero - posterior and 

 dorso - ventral axes. After 

 Lanlcester — Zoological Articles 

 reprinted from the Enoyclo- 

 pcedia BHtannica, 



I. A Lamellibranch. 

 II. AuAnisopleurousGasteropod. 

 III. A Cephalopod. 

 A. Anterior surface. 

 P. Posterior surface. 

 D. Dorsal surface. 

 V. Ventral surface. 



1. Mouth. 



2. Anus. 



3. Mantle cavity. 



4. Foot. 



is protruded, but come together when that organ is withdrawn, 

 by means of these the Natantia, which are carnivorous, catch 

 other pelagic organisms to feed upon. 



Von Erlanger has shown in the developement of Paludina, 

 that the glandular portion of the nephridium arises as an 

 evagination of the pericardium, and the mouth of the evagin- 

 ation remains as the reno-pericardial pore, when such an orifice 

 exists. The genital gland arises as a proliferation from the 

 pericardial wall at a spot where a rudimentary evagination has 

 been formed which represents the missing nephridium. The 

 duct of the nephridium and the duct of the gonad are sym- 

 metrically situated and homologous involutions of the epiblast. 



