No. 447.] 



HAMIXEA SOLITARIA SAY. 



213 



there are two tentacular disks, which are divided only part way. 

 There is very little movement in these disks, except as they 

 shift about over the anterior portion of the shell. In this species 

 there seem to be no special sense organs in the tentacular disks. 

 So far, I have been unable to explain their function. The 

 tactile sense is usually located in the tentacles, but in our 

 animal the anterior jjortion of the snout performs this function, 

 as is very evident from its movements. In many of the Opis- 

 thobranchia the mantle folds over part of the shell, but in //. 

 solitaria the only external evidence of the mantle is a thick fold 

 which occupies the lip of the shell and extends back of it for 

 about four mm., it is here extended and affords a surface equal 

 in width to the shell. At first sight one would think because of 

 this arrangement that the foot extended from the snout to the 

 posterior portion of the animal, but closer inspection shows a 

 sharp demarcation which indicates the boundary between the 

 posterior portion of the meso-podium and the mantle. All of 

 the parts of the body that are exposed are covered with colum- 

 nar, ciliated epithelium. A great quantity of mucus is secreted 

 by the animal, so that in crawling about the dish they often 

 leave a nearly perfect tube of mucus. This is secreted by 

 numerous typical single-celled glands, which are esi)ecially abun- 

 dant at the edges and tip of the snout, and the outer portion of 

 the tentacular chsks. 



The especial characters aside from the shell which distinguish 

 H. solitaria are the stomach plates and 

 the form of the radula. 



There are three stomach plates so 

 arranged that the food is trituated by 

 them (Fig. 3). The plates are com- 

 posed of chitin, having their bases 

 firmly imbeded in the strong muscles 

 of the stomach. The form of a single y,,,. 3. -showing the arrange- 

 plate is shown by the camera drawings J";°/j|'^"|^^,dd^^^^^^ 

 (Figs. 4, 5). The portion of the plate J'"^""*"'^;^'^^;^^^^^,^;^^^^^^^^ 

 that comes in contact with the food is It^perf^S tjTfTd. '° 

 differentiated into a number of ridges 



which are larger and more pronounced at the anterior end. The 



