CEPHALIC TENTACLES. 
187 
towards the base of the oiiiinatophore of its side, both main chaiiiiels 
afterwards becoming rather regularly tritid before their final oblitera- 
tion. In Limd.v maxlmus these light coloured grooves or mucus 
channels are nincli simpler in character, each dorsal furrow merely 
giving off one slender branch towards the ommatophore of its side. 
All our Gastropoda do not, however, possess these distinctly detined 
ramified channels, as I have been (luite unable to detect recognizable 
indications of their presence in many species. 
The Cephalic Tentacles, which arise during larval development 
from the area encircled by the velum, are pre-eminently sensitive, 
tactile and symmetrically paired processes of the body wall, and are 
always innervated by the cephalic ganglia, and placed on the antero- 
dorsal aspect of the animal. They are the most noticeable and prominent 
appendages of the ce})halic region, being very mobile and possessing 
a considerable variety and range of motion, and externally covered in 
the terrestrial sj)ecies with fine granulations or annular ridges, and 
in the acpiatic species with a veiy noticeable vibratile epithelium. 
They may be divided, according to their structure, into two groups, 
viz. : Retractile and Contractile. The retractile tentacles are hollow 
and muscular cylindrical processes, each terminated by a bulbous en- 
largement, and capable of being withdi’awn within the body of the 
animal and are more especially a characteristic of the terrestrial 
species. The contractile tentacles, for which Ehrenberg proposed the 
term Vibracles to distinguish them from the retractile tentacles of the 
Helices, are solid, homogeneous and projecting cephalic processes, 
only capable of shrinkage or contraction, and are quite incapable of 
being withdrawn within the body cavity like the retractile tentacles 
of the Stylommatophores. 
The Stylommatophores have usually two pairs of elongate, cylin- 
drical and tapering hollow tentacles, which are filled with blood and 
connected with the blood spaces of the head, and bear a well-develo])ed 
bulbous distal extremity. The dorsal or posterior pair, in addition 
to olfactory organs, bear the eyes at the bulbous end, being hence 
termed Ommatophores or eye-bearers, and are usually three or four 
times longer than the anterior jiair, which, however, are very similar 
in general organization. The anterior tentacles, however, vary greatly 
in their development ; in Buliminus and Biqxi they are very short, 
and in Carychium quite rudimentaiy, while in Vertigo no external 
trace of their existence can be detected. 
