126 MOLLUSCA. 



Class III. Pteropoda. 



(Form a transitional group to Cephalopoda.) 



Marine hermaphrodite Molluscs, devoid of ctenidia, with the mid-foot 

 produced into a pair of wing-like fins. Visceral hump large, and ex- 

 hibiting secondary bilateral symmetry ; anal, genital, and nephridial 

 apertures anterior and lateral. 



Order 1. Thecosomata, with shell; the fins not sharply marked off; 

 a mantle-cavity, often containing branchial folds : Hyalea tridentata, 

 Cymbulia, Tentaculites (fossil). 



Order 2. Gymnosomata, without shell ; fins sharply marked off ; no 

 mantle-cavity. Tentacles on the head. Larva with a zone of cilia: 

 Clio borealis. Pneumodermon, two arms covered with suckers. 



