ORDER II CTENOBRANCHIATA—HETEROPODA—PLATY PODA 455 
Order 2. CTENOBRANCHIATA. Schweigg. 
(Pectinibranchia, Cuvier ; Azygobranchia, Ihering ; Monotocardia, Bouvier.) 
Right 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 Ctenobranchiata constitute the largest group of the Streptonewra. 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-drders primarily : 
Platypoda, 1 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 Twendoglossa. 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- is 
shaped, glassy shell; while in Atlanta, Lesson (Fig. 860), the Beede Mee 
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. 

Fic. 860. 
Sub-Order B. PLATYPODA. 
Superfamily 1. GYMNOGLOSSA. 
More or less completely parasitic forms, in which the radula is unarmed through 
degeneration. 
