3 z THE POPULAR SCIENCE MONTHLY. 



The possession, by many members of this class, of two fin-like mus- 

 cular expansions attached to the side of the head induced Cuvier to 

 give them the name Pteropoda. Prof. Owen says : "All the species 

 of Pteropoda are of small size ; they float in the open sea, often at 

 great distances from any shore, and serve, with the Accdep/ice, to 

 people the remote tracts of the ocean. In the latitudes suitable to 

 their well-being, the little Pteropoda swarm in incredible numbers, so 

 as to discolor the surface of the sea for leagues ; and the Clio and the 

 Limacina constitute, in the northern seas, the principal article of food 

 of the great whales." 



Some of the least highly-organized members of this class, such as 

 the Hyalaceos, are provided with a bivalve shell, and cannot be said 

 to possess a head. They have a simple commencement of the ali- 

 mentary canal at the anterior extremity of the body ; but since this 

 anterior extremity has no tactile appendages and no eyes, and inas- 

 much as it also contains no cerebral ganglia, it can have no claim to 

 be considered as a head. Their chief nervous centre consists of a 

 flat, somewhat quadrate, sub-oesophageal ganglion, to the anterior 

 angles of which is attached a nervous commissure which extends 

 upward so as to encircle the gullet, though there are no ganglia either 

 on or at the sides of this tube in the usual situation occupied by cere- 

 bral ganglia. 



In other pteropods devoid of a shell, we meet with a higher organ- 

 ization. Thus in Clio there is a distinct head bearing sensory ap- 

 pendages in the form of two tentacula and two eyes, and containing 

 in its interior a brain. This brain is represented by two connected 

 super-oesophageal ganglia, which are in relation, by means of nerves, 

 with the cephalic sensory organs, and in connection with the sub-ceso- 

 phageal commissure are the two pedal and two branchial ganglia. 

 The two pairs of ganglia exist separately in Clio and its allies, 

 though they are combined into one quadrate mass in Hyalea. In this 

 latter there are two acoustic vesicles in contact with the anterior part 

 of the great ganglion, while in Clio similar vesicles are in connection 

 with the anterior pair of sub-cesophageal ganglia that is, with the 

 pair which corresponds with the pedal ganglia of the common bivalve 

 mollusks. 



Gasteropods constitute a class of organisms which, in point of 

 numbers, can only be compared with the still more numerously repre- 

 sented class Of insects. Their name is derived from the fact that 

 these animals crawl by means of a large muscular expansion stretched 

 out beneath the viscera. The locomotion of the members of this class 

 may be said to be, in the main, dependent upon their own individual 

 efforts, so that, in this respect, they differ widely from the pteropods, 

 whose locomotions are brought about by wiuds driving them along 

 the surface of the water on which they float. 



Some gasteropods are terrestrial, air-breathing animals, though by 



