102 COMPARATIVE ANATOMY CHAP. 



tions for seizing the prey. We thus have a combined head and foot 

 (Kopffuss), on each side of which, anteriorly, lies a large highly- 

 developed eye. This head is more or less separated from the rest 

 of the body (the visceral dome) by a neck. 



The Gastropoda, with very few exceptions, possess a head which on 

 its anterior lower side is provided with an oral aperture, on its upper 

 side with eyes and tentacles, and often asymmetrically (generally on 

 the right side) with a genital aperture or a copulatory organ. This head 

 is distinctly separated ventrally by means of a groove or furrow from 

 the foot behind it ; dorsally it passes gradually into the neck. Further 

 details of this Gastropod head are given below. 



A. Gastropoda. 



1. Prosobranchia. 



The head in this order always carries tentacles, which are solid, simply contractile 

 (not invagmable) processes of the cephalic wall. It may be assumed that there 

 were originally two pairs of tentacles, an anterior and a posterior pair. The 

 posterior are called ommatophores and carry eyes at their tips. Most Diotocardia 

 possess anterior tactile tentacles, and posterior and slightly lateral optic tentacles. 



The cephalic tentacles are always innervated from the cerebral ganglion, and are 

 thus distinguishable from the tentacular processes which may occur near them on 

 the head or neck, but belong to the epipodium, and are innervated from the pedal 

 or pleural ganglia. 



In the Docoglossa and most Monotoeardin the optic tentacles do not rise separately 

 from the head, but are to a greater or lesser extent fused with the tactile tentacles. 

 Starting with the tentacular arrangements existing in Dolium, S trombus, Rostellaria, 



we find the tactile and optic tentacles 

 fused for a certain distance from tin- 

 base, but separating later, the tips 

 projecting independently (Fig. 96, B). 

 If the two tentacles were of the 

 same length, and were fused for their 

 whole extent, there would only be one 

 F,r, 90. -Relations of the tactile and optic tentac i e on each side of the head, which 

 tentacles in the Prosobranchia. Description in 

 the tpxt would carry the eye at its tip (Tereora 



(7). But if the optic tentacle is shorter 



than the tactile, the eye might be met with at any point between the base and tip 

 of the latter, on a projection which answers to the tip of the fused optic tentacle (D 

 and E). Finally, the eye may be altogether sessile, i.e. it may lie near the base of 

 the sensory tentacle in the integument of the head (F). 



The snout, which carries the mouth and is anterior to the tentacles, is very 

 variously developed in the Prosobranchia. 



1. It is short and truncated in the Diotocardia, and especially in the herbivorous 

 Tcvnioglossa, 



2. It is prolonged like a proboscis (rostrum), but is only contractile, not invagm- 

 able (Capulidcc, Strombidce, Ctcnopidcc, Calyptrceidce), or else can be invaginated, 

 commencing at the tip (C '///"" ''"V'- 1 , Lamellaridix, Natiddce, Scalarida', Solaridce). 



3. It is transformed into a long proboscis with the mouth at its anterior end. 

 This proboscis can be invaginated in such a way that the invaginated base forms a 

 proboscidal sheath for the non-invaginated anterior portion or tip. Gastropods 



