494 
PAUL PELSENEER. 
not seen any special nervous terminations, such as we shall 
find on the buccal cones. 
Posterior Pair. — It is situated on the dorsal face of the 
neck. These tentacles are much shorter than those of the first 
pair. Upon the animals preserved in spirit they are always 
retracted, so that they are difficult to see, their presence being 
then disclosed only by two slight recesses, from which their 
extremities are sometimes seen emerging under the form of a 
white point. 
According to Eschricht, this second pair is oculiferous ; 1 but 
this fact has been called in question by several naturalists, 
and categorically denied by von Ihering . 2 
The histological examination which I have made of the 
nuchal tentacles of Clione, Clionopsis, and Pneumo- 
derm on, allows me to assert that these appendages do present 
eyes at their free extremity. 
Unfortunately the specimens which have served me in my 
researches had not been specially prepared for the study of the 
nervous system and of nervous terminations as delicate as the 
retinal ones. It follows that I can neither draw nor describe in 
a complete manner the structure of the eyes of the Gymnoso- 
mata. That is a point which I intend to take up as soon as 
possible. 
Nevertheless, the profound dissimilitude which I have 
observed between the labial and nuchal tentacles, and the 
characters of the structure of the latter in the three species 
which I have studied, show that the nuchal tentacles are quite 
different from those of the first pair, and the presence of a 
refracting body shows that the sense of which they are the seat 
is that of sight. 
At the free extremity of these tentacles, the epithelium 
becomes much thinner, so as to make a pellucida or cornea. 
Under this membrane we find a spherical lens, of which the 
structure is similar to that of the Pulmonata. As I have 
1 Eschricht, loc. cit., taf. 3, fig. 29. 
1 Von Ihering, ‘ Vergleichende Anatomie der Nervensystem und Phylogenie 
der Mollusken,’ p. 240. 
