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 
Fia. 122.—Diagrams of a series of 
Molluses 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 
Lankester—Zoological Articles 
reprinted from the Hncyclo- 
pedia Britannica, 
I, A Lamellibranch. 
II, An Anisopleurous Gasteropod. 
III. A Cephalopod. 
A. Anterior surface. 
P. Posterior surface. 
D. Dorsal surface. 
V. Ventral surface. 
. Mouth. 
Anus. 
Mantle cavity. 
Foot. 
Lo a LSI 
is protruded, but come together when that organ is withdrawn, 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. 
