THE PTEROPODA. 435 



spinose tentacular organs are developed at the sides of the 

 mouth, and, in addition, two acetabuliferous tentacles take 

 their origin on the inner side of a cup-like hood, which sur- 

 rounds the anterior end of the body. 1 Cymbulia is stated 

 to possess no radula. The epipodia are large muscular ex- 

 pansions, by the flapping of which the Pteropods swim ; but 

 the rest of the foot is always small, and often rudimentary, 

 in correspondence with the small size of the neural face of the 

 bodv. 



The haemal face, on the contrary, is always produced, as 

 in the Cephalopoda, into a relatively large visceral sac ; and 

 in some (the Thecosomata) this visceral sac is coextensive 

 with the mantle, which is protected by a shell. In others 

 ( Gymnosomata) the mantle early disappears, and there is 

 no shell. In Cymbulia, the delicate transparent chitinous 

 shell is internal, and is invested by an epithelial layer derived 

 from the mantle. In Spirialis, the foot bears an operculum. 

 Chromatophores similar to those of the Cephalopoda occur in 

 Tiedemannia. 



In the Thecosomata, the free lcbe of the mantle, which 

 incloses a spacious pallial cavity, usually lies on the posterior 

 aspect of the visceral sac, as in the Cephalopoda, and the 

 rectum terminates in it, on one side of the middle line. In 

 these there is a simple neural flexure of the alimentary canal, 

 as in the Cephalopods, although the turning of the rectum to 

 one side destroys the symmetry of the body. In Limacina 

 and Sfjirialis, the intestine appears to be bent round to the 

 anterior face of the visceral sac, the mantle-cavity accom- 

 panying it, so that the opening of the mantle is placed on 

 the anterior, instead of on the posterior, face of the visceral 

 sac. There are no distinct gills in the Thecosomata, but the 

 lining of the mantle-cavity subserves the function of respira- 

 tion, and is sometimes produced into folds, which doubtless 

 aid in the performance of that function. Processes of the 

 bcdy, to which the office of gills is ascribed, are found in 

 some Gymnosomata {Pnewmodermo?i ISpongooranchia). 



The heart consists of a single auricle and a single ventricle. 

 The auricle lies close to the pallial cavity, and receives the 

 aerated blood from its walls. The ventricle is sometimes 

 directed forward (as in all Gymnosomata), and sometimes 



1 See, for the somewhat similar arrangements in Clione, Eschricht, " Ana- 

 tomische Untersuchungen Tiber Clione boreaHs," 1858; and Macdonald, "On 

 the Zoological Characters of the Living Clio czudata." (" Trans. Royal Society 

 of Edinburgh," 1863.) 



