LECTURES ON MOLLUSCA. 91 



or fine, soft hairs, spread over the surface of the body ; and by plaits 

 or vessels radiating on the back. Their bodies have long swimining 

 flaps ; and the branched liver-vessels open into the sides of the 

 stomach. 



Family LIMAPONTIAD.E. 



In these lowest of Opisthobranchs, as in the lowest of the Hetero- 

 pods, there are no special breathing organs. The aeration of the 

 blood is carried on entirely through the skin. In general appearance, 

 these creatures are like lungless Slugs. In Limapontia and ActcKonia, 

 the tentacles are crest-like ; in Ictis, Fucola, and Pelta, they are 

 linear. The little genus Rlwdope is like a creeping worm, without 

 mantle, shell, gill, tentacle, or any other appendage. It appears the 

 most degraded of Crawlers, but no doubt enjoys life in its own way as 

 it progresses over the sea-weeds of Messina. 



SUB-CLASS HETEROPODA. 



The Heteropods, or Nudeobranctis as they are sometimes called, 

 are a very aberrant race of creatures ; and, as such, placed in very dif- 

 ferent positions by naturalists. They are in fact Gasteropods, adapted 

 for swimming in the open seas. As they do not crawl on the belly, 

 they have scarcely a right to the name of the class : accordingly some 

 authors treat them as an independent division, between the Gastero- 

 pods and the Pteropods. As however we have seen the crawling foot 

 obsolete in the stationary Magilus and Vermetus ; and scraggy, more 

 fitted for leaping, in Strombus and Phorus ; it is no great strain on 

 our general idea of a Gasteropod to imagine its foot flattened into a 

 fin for flapping in the open sea. Many of the Opisthobranchs have the 

 foot developed into side-flaps for swimming : we have only now to 

 imagine the boat propelled by one central scull instead of by a pair of 

 oars. It appears the simplest arrangement to regard them as a group 

 coordinate with the crawling Gasteropods, but inferior to them ; as the 

 implacental by the side of the ordinary Mammalia. 



In some respects the Nucleobranchs are superior to the ordinary 

 crawlers. Their bodies are more symmetrical and their locomotion 

 more active. Dr. Gray, indeed, arranges lanthina with Scalaria among 

 the Proboscidifers, and the remaining groups with the Rostrifers. 

 Nevertheless, the lower tribes are so like the lower tribes of Nudi- 

 branchs which indeed they all resemble in the exposure of their gills ; 

 and the whole group forms so natural a transition to the Pteropods, 

 that this appears their most appropriate place. It will be understood, 

 however, that Nature never arranges her creations in straight lines ; 

 but the higher animals in one division are commonly more complete in 

 organization than the lower animals in the groups above it : each type 

 producing the highest as well as the lowest within its own sphere. 



The Heteropods have the sexes distinct, like the Comb-gilled Crawl- 

 ers ; and, like them, have the gills in advance of the heart. They 

 resemble the Tectibranchs in the subordination of the shell ; which 

 sometimes envelopes the whole animal, sometimes only the vital organs, 



