70 OUTLINES OF NATURAL HISTORY. 



sides there are two slits, one on each side, throngli which 

 pass long appendages of the integument or " mantle." 



The animal is attached to the interior of its shell by a 

 muscle which passes from the point of the shell behind to 

 the head in front. On each side of the head is situated a 

 large fin (fig. 30, //), by means of which the animal 

 propels itself through the water. The head is also fur- 

 nished with indistinct tentacles, and exhibits centrally, on 

 its front margin, the opening of the mouth. The mouth 

 contains a toothed " tongue," essentially similar to that of 

 the \Mielk. The mouth opens into a long and slender 

 gullet, which conducts to a stomach, this in turn com- 

 municating with a long and slender intestine. There is a 

 well-developed liver, and the intestine terminates in a dis- 

 tinct vent placed on the right side of the neck. The ner- 

 vous system forms a mass situated below the gullet. There 

 is a heart, consisting of two chambers ; but the breathing- 

 organs are quite rudimentary, and can hardly be said to 

 constitute regular gills. 



Hycdea tridentata is a native of the ]\Iediterranean and 

 Atlantic Ocean, and is found in the open sea, far from 

 land, darting about by means of the vigorous flapping of 

 the lateral fins. It appears to be of nocturnal habits, and 

 to sink below the surface during the daytime. 



Recapitulation of Essential Characters, -r— The 

 head is furnished with lateral expansions of the skin, 

 or fins, by means of which locomotion is effected. The 

 mouth is furnished with a toothed tongue. The animal 

 lives in the open ocean, near the surface of the water. A 

 non-essential but common character is, that the body is 

 protected by a symmetrical, glassy, semi-transparent shell. 

 These characters di.^tinguish the class of the Pteroiioda as 

 a whole. 



Historical Collec.ion 



