THE ANATO.Ary OF PLEUROTOMAEIA BEYeiOHlI. 221 



The Head. — The head is large aud produced into a 

 somewhat cylindrical snout, bent downwards as in the Tro- 

 chidae. On the under surface, which is abruptly truncated, 

 is situated the mouth, surrounded by a horseshoe-shaped lip, 

 the gap in the lip being placed ventrally. The head and 

 anterior part of the body are much smoother than the foot, 

 being practically devoid of papillae. The tentacles are situated 

 low down on either side of the head. In form they are cylin- 

 drical, with roughly pointed extremities ; they are highly mus- 

 cular and evidently much contracted. The only peculiar 

 feature about them is the tendency which they show to be- 

 come branched at their free ends; this is most marked in the 

 left tentacle of the first specimen examined (fig. 1), but the 

 second specimen also showed indications of a similar condi- 

 tion in both tentacles, although to a lesser degree, while the 

 right tentacle of the third specimen shows no less than two 

 accessory branches. 



The Eyes. — The eyes, which are small and inconspicuous, 

 are situated, each on a slight elevation, at the posterior side of 

 the base of the tentacle. Examination with a lens shows the 

 cornea to be perforated, as in Trochus and most Dioto- 

 cardia. 



An examination of sections (fig. 20), however, shows that 

 they are simple in constitution; like those of Trochus the 

 central cavity is only partially filled by a vitreous body, the 

 rest of the space being occupied by sea-water. 



The eyes, as may be supposed, are not specially well pre- 

 served, but one can see that the retina consists of a series of 

 pigmented rods, turned towards the optic cup, and an external 

 layer of ganglionic cells. I was unable to make out the clear 

 distal segments of the rods, such as are figured for other 

 Diotocardia. The retina is bounded by a delicate capsule, 

 outside which we see the optic nerve aud a few mesodermal 

 pigiuent cells. The retinal pigments extend out through the 

 perforation in the optic cup into the adjacent epidermis. 

 The structure of the vitreous body suggests that is secreted 

 by the individual rod-cells. 



