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MANUAL OF ZOOLOGY. 



CHAPTER XLIX. 

 PTEROPODA. 



CLASS III. PTEROPODA. The Pteropoda are defined by being 

 free and pelagic, swimming by means of two wing-like appen- 

 dages (epipodia), developed from each side of the anterior 

 extremity of the body. The flexure of the intestine is neural. 



As to the position of the Pteropoda in the Molluscan scale, 

 they must be looked upon as inferior in organisation to any of 

 the Gasteropoda, of which class they are often regarded as the 

 lowest division. They permanently represent, in fact, the 

 transient larval stage of the sea-snails. 



The Pteropods are all of small size, and are found swimming 

 at the surface of the open ocean, often in enormous numbers. 

 Locomotion is effected by two wing-like fins, developed from 



Fig. 142. Pteropoda. a. Cleo'Jora pyramidata : b Cuvieria coliunnella, 

 (After Woodward.) 



the sides of the head, and composed of the greatly developed 

 " epipodia." The true " foot " is rudimentary and rarely dis- 

 tinct, but the " metapodium " is sometimes provided with an 

 operculum. There is usually a symmetrical glassy shell (fig. 

 142), either consisting of a dorsal and ventral plate united, or 

 forming a spiral (fig. 135, B), but in some cases the body is 

 naked. The head is rudimentary, and bears the mouth, which 

 is occasionally tentaculate, and which is furnished with an 

 odontophore. There is a muscular stomach and a well- 

 developed liver ; and the flexure of the intestine is neural, so 

 that the anus is situated on the ventral surface of the body. 



The heart consists of an auricle and ventricle. The re- 

 spiratory organ is very rudimentary, and consists of a ciliated 

 surface, which is either entirely unprotected, or may be con- 

 tained in a branchial chamber. 



The ganglia of the nervous system " are concentrated into 



