GASTEROPODA PECTINIBRANCHIATA. 



247 



ORDER V. 



HETEROPODA. 



THE Heteropoda are distinguished by their foot, which, instead of forming 

 a horizontal disk, is compressed into a vertical mus- 

 cular lamina, which they use as a fin, and on the 

 edge of which, in several species, is a dilatation 

 forming a hollow cone, that represents the disk of 

 the other orders. Their branchiae, composed of 

 plumiform lobes, are situated on the hind part of the back, directed forwards, 

 and immediately in their rear are the heart and a small liver, with part of the 

 viscera. Their body, a gelatinous and transparent substance lined with a mus- 

 cular layer, is elongated and usually terminated by a compressed tail. There 

 is a muscular mass belonging to the mouth, and a tongue furnished with 

 little hooks. They usually swim on their back with the foot upwards. They 

 have the faculty of distending their body by filling it with water, in a way not 

 well understood. Forskahl comprised them all in his genus 



PTEROTRACHEA. 



Which is now divided into Carinaria, Atlanta, Firola, Timorienna, Mono- 

 phora, Phylliroe. The Argonauta vitrea of authors must be the shell of a 

 Carinaria, but the animal is not yet known. 



ORDER VI. 



PECTINIBRANCHIATA. 



THIS order forms, beyond all comparison, the most numerous division, inas- 

 much as it comprises the whole of the spiral univalves, and 

 several that are simply conical. Their branchice composed 

 of numerous lamellae or strips laid parallel with each other, 

 like the teeth of a comb, are attached on one, two, or three 

 lines, according to the genus, to the ceiling of the pulmonary 

 cavity, which occupies the last whorl of the shell, and which 

 has a large opening between the edge of the mantle and the 

 body. 



In two genera only, Cyclostoma and Helicina, do we find; 

 instead of branchiae, a vascular net- work, covering the ceiling 

 of a cavity, that is otherwise similar ; they are the only ones that respire the 

 natural air ; all the others respire water. 



All the Pectinibranchiata have two tentacula and two eyes, sometimes placed 



