ANIJIAL — HYPOTHETICAL PRIMITIVE TYPE. 
U2 
accessoiT organs, and is a later and more complex development, 
apparently intlnenced by the adoption of terrestrial or flnviatile life, 
parasitism, tixed habits, etc. 
The Hypothetical Primitive mollusk, from which all the varied 
forms now existing have been derived, has been assumed, with great 
probability, to have been a bilaterally symmetrical animal, which 
possessed in addition to other general characters a well marked 
locomotor foot or muscnlar creeping 
disc and a more or less well-defined 
head, bearing a ventral month and 
])aired tactile processes or tentacles, 
at the base of which were probably 
pigmented eye spots. Dorsally the 
animal was overspread by a fold of 
the integument, called the mantle, 
which hnng loose at the margins 
around the body, forming a space or cavity between its free pendnlons 
margin and the body-wall- the pallial cavity, within which were the 
paired feather-like gills and the excretory outlets. This integnment 
had the physiological jiower of secreting a more or less calcareous 
shell, with a chitinons frameivork to which the retractor muscles of 
the body were tirmly attacheil. 
The nervons system exhibited the archaic character of long paired 
nerve cords, with ganglionic enlargements on each side of the median 
line, arising from the paired snpra-msoijhageal ganglia and connected 
together beneath the alimentary canal or digestive tract, upon the 
course of which various ferment glands are developed. The auditory 
capsules or otocysts are i)laced in the anterior part of the foot, but 
innervated by the cephalic ganglia. d'he olfactory organs are sensi- 
ferous areas near the base of each branchia, which receive their nerve 
su])ply from the ])allial ganglia. The gonads or genital glands are 
])resent as dependencies of the iiericardial chamber, their i)roducts 
reaching the exterior by the reno-pericardial orifices. 
The circulatory organs were confined within the secondary cadomic 
cavity or i)ericardium, the heart however being probably i)riniitively 
doulde, as suggested by Dali for the Protopelecyjiod, with a ventricle 
and an auricle on each side, the ventricles eventually fusing in the 
meilian line and, in some groups, enclosing the rectum, 
Fig. 301. — Hypothetical Primitive Mollusk, 
showing e.xtenialiy the dorsal shell, paired 
tentacles and over-hanging mantle margin 
and internally the alimentary canal, heart, 
branchia and genital glands, with longitudinal 
untwisted nerve cords and ganglionic en- 
largements (after Pel.seneer). 
