SECT. XI. 



PTEROPODA. 



243 



motion. This little animal is highly organized ; it has a 

 gullet, a kind of crop and gizzard, a liver, a respiratory- 

 tube, a heart, a circulating and nervous system, which 

 enables it to swim with a flapping motion of its fins. 



Fig. 177. A, Hyalaea; B, Clio. 



The Clio pyramidata (B, fig. 177) is an elegant animal 

 belonging to the same class. Its fragile transparent 

 shell has the form of a triangular pyramid ; and from 

 its base proceeds a slender spine, and a similar spine 

 extends from each side of the middle of the shell. The 

 posterior part of the body is globular and pellucid, and 

 in the dark it is vividly luminous, shining 

 through the glassy shell. The fins of the 

 Hyalsea and Clio or Cleodora are of a bright 

 yellow, with a deep purple spot near the 

 base. Both are inhabitants of the ocean. 



The Clione borealis (fig. 178), which 

 exists in millions in the Arctic Seas, is 

 the most remarkable instance of the 

 Naked Pteropods. It has neither shell 

 nor mantle; its membranous body is not 

 more than half an inch long, its head is 

 formed of two round lobes, on each side 

 of the neck there is a large muscular wing 

 or fin ; in swimming the animal brings the tips of the 

 fins almost in contact, first on one side of the neck and 



B 2 



