ORDER ii CTENOBRANCHIATA HETEROPODA PLATYPODA 455 



Order 2. CTBNOBRANCHIATA. Schweigg. 



(Pectinibranchia, Cuvier ; Azygobranchia, Ihering ; Monotocardia, Bouvier.) 



Eight cervical gill pectinate, very large, and usually transposed to the left side, owing 

 to torsion of the body ; the left gill atrophied. Heart with but one auricle. Radula 

 small, variously constructed, but usually armed with few teeth in a transverse series. 

 Shell coiled in a more or less elevated spiral, rarely cup- or cap-shaped. 



The Gtenobranchiata constitute the largest group of the Streptoneura. They are 

 for the most part marine, but some are terrestrial, and some inhabit fresh water. 

 Beginning in the Silurian, they attain their maximum distribution in the Mesozoic, 

 Tertiary, and Recent periods. A division into two groups Holostomata and Siphono- 

 stomata according to the nature of the aperture, has been attempted ; but this is 

 unnatural, since it emphasises a shell character which is unaccompanied by any 

 anatomical differences. Classifications based upon the structure of the radula, such as 

 have been proposed by Troschel, and more recently by Bouvier, are valueless in 

 Palaeontology. Here it will be sufficient to recognise two sub-orders primarily : 

 Platypoda, in which the foot is normally developed ; and Heteropoda, in which it is 

 modified into a fin. 



Sub-Order A. HETEROPODA. Lamarck. 



(Nucleobranchiata, Blainville.) 



To the Heteropoda belong naked or shell-covered, free -swimming and pelagic 

 marine Mollusks, with distinct head and highly developed sense organs. Heart, gills, 

 reproductive organs, and nervous system agree with the corresponding organs of the 

 Ctenobranchiates ; the radula resembles that of the Taenioglossa. They differ con- 

 siderably, however, from the Prosobranchiates, since the foot is modified into a sort of 

 vertical fin, and imparts to them a peculiar appearance. They rise usually toward 

 evening in great swarms to the surface of the ocean, where they 

 hover about with a very rapid motion, swimming in an inverted 

 position, with the dorsal side down, and the foot uppermost. 

 They are exceedingly delicate, often transparent organisms. 

 The body may be either entirely naked or provided with a 

 very thin, light shell. 



Two recent genera have been found also in early Tertiary 

 deposits. Of these Carinaria, Lamarck, has a keeled, cap- 

 shaped, glassy shell ; while in Atlanta, Lesson (Fig. 860), the 

 delicate shell is coiled spirally in a single plane, and the 



aperture is provided with a slit. Owing to the great similarity of Atlanta and 

 Oxygyrus to certain Palaeozoic Bellerophontidae, a relationship between the two has 

 been not unplausibly suggested. The latter forms are distinguished principally by 

 their heavier, thicker shells. 



Sub-Order B. PLATYPODA. 



Superfamily 1. GYMNOGLOSSA. 



More or less completely parasitic forms, in which the radula is unarmed through 

 degeneration. 



