CEPHALIC APPENDAGES OF GYMNOSOMATOUS PTEEOPODA. 495 
already said, the retinal part is not perfect in any section, but 
its position and general form recall to mind those of the retina 
of the Gastropoda. In the specimens of Clione the pigment 
had disappeared, but in those ofPneumodermon it was pre- 
served. Finally, towards the base of the tentacle the nerve 
traverses an optic ganglion. 
b. Buccal Cones. 
They number three pairs, symmetrically situated on the two 
sides of the “ lips.” The two dorsal cones are the longest, the 
two ventral cones the shortest (fig. 3). 
These cones are very extensible, and in the state of expansion 
they are much longer than they are generally represented. 
They are absolutely conical, as my figures show them, which I 
made from specimens 1 of the “ Challenger,” preserved with the 
extended cones and all the appearances of the living animal. 
These cones are brightly coloured during life. They are 
inserted on the two sides of the “ lips ” as shown in fig. 4. 
They are hollow in their lower half, and their cavity is con- 
tinuous with that of the head, which includes the buccal 
mass and the penis. Examining these cones with a magni- 
fying glass, one sees that they are covered with innumerable 
very small tubercles, which have been described by Eschricht 
as so many groups of suckers. 
Structure of the Buccal Cones. 
On any section of a buccal cone of Clione we may easily 
distinguish three different regions : 
1. A middle, muscular region formed of two parts: a. an 
exterior layer with annular fibres (fig. 11, a); b. an interior 
layer with longitudinal fibres (fig. 11, a'). 
2. An internal region formed of glandular cells (fig. 11, b). 
3. An external region, or epithelial clothing (fig. 11, c). 
1 Mr. John Murray, F.R.S.E., Director of the Challenger publications, 
kindly supplied Professor Lankester with these specimens as soon as he heard 
that Clione was undergoing investigation in the laboratory of University 
College. 
