242 LECTURES ON MOLLUSCA. 



have no gills or fins; being simply a floating, gelatinous, slug-like 

 body, with long tentacles but no eyes. In the union of sexes, the 

 teeth, and the digestive organs, they resemble the Nudibranchs; in 

 their habits and general appearance the Heteropods. They breathe 

 all over the skin, like the lower species of Firoloidea. The tail of 

 Phylliroc is flattened into a fin ; that of Acura is pointed. 



Family Pterosomatid/E. 



The curious little bit of jelly which comjDoses this family may be 

 compared to a thin Acura, with eyes instead of tentacles, but no snout; 

 laid on the middle of a broad, floating flap. Its anatomy is not yet 

 made out; but it forms a transition to the Pteropods. 



CLASS PTEROPODA. 



( Wing-footed Mollushs.) 



The "Sea-Butterflies," as they are sometimes called, are a race of 

 creatures formed to live, permanently, swimming about in mid ocean. 

 They are recognized at once by the two delicate fins, which are con- 

 stantly moving, with considerable animation, when at the surface of 

 the water. Most of them are crepuscular or nocturnal in their habits ; 

 spending the day, poised in the lower depths, and rising, at different 

 periods and degrees of darkness, according to the species, to enjoy their 

 active life. Some kinds, however, disport themselves beneath the mid- 

 day tropical sun. In their first stage, they exactly resemble the fry of 

 the Grasteropods ; but the larval fins of the Pteropods fall off, like those 

 of their neighbors, and the permanent fins are developed round the 

 neck, answering perhaps to the neck-lappets of the Turbos, &c. They 

 have no foot ; but in some of the groups there is a little lobe between 

 the fins, which is its commencement. Sometimes their feelers have a 

 few minute suckers, by which they can hold their prey or moor them- 

 selves to floating objects; in w r hich, and in the bending back of the 

 alimentary canal along the abdomen, they resemble the Cephalopods. 

 They are however inferior, in point of organization, to the Crawlers. 

 They have a very feeble circulation and respiration ; the nervous centres 

 are behind the gullet ; there are no eyes ; the gills either do not exist 

 or are near the tail ; and the senses are rather diffused over the body 

 than localized in special organs. In the reproductive system, and in 

 many special points of structure, they closely resemble the Heteropods. 

 In fact, it is probable that the whole class of Pteropods should be re- 

 garded simply as a subclass of Gasteropods, connected with the typical 

 forms by Carinaria and Ianthina. Like the Heteropods and Opistho- 

 branchs, some have shells and others none; but in this tribe, the shelly 

 races are the lowest in rank, inasmuch as they have no heads: in this 

 respect alone passing into the next great group of bivalves. They arc, 

 therefore, here arranged after the naked tribes. 



The Pteropods are few in number, as far as species are concerned; 

 but these are widely diffused, may of them being common to the At- 

 lantic and Pacific oceans. But in individuals they are incredibly 



