Gastropod <t . 

 Sub-class 2. EUTHYNEURA. 



31 



The Gastropods belonging to this sub-class, with few exceptions, 

 have the visceral nerve-loop uncrossed. All the Euthyneura are 

 hermaphrodite, and their radula is generally composed of 

 numerous .similar denticles on each side of a median tooth. 

 Scarcely any are provided with an operculum in the adult state. 

 The Euthyneura may be divided into two orders, Opisthobranchia 

 and Pidmonata. 



Order 1. OPISTHOBRANCHIA. 



All the molluscs of this order are marine, some {Tectibranchia) 

 breathing by means of the ordinary Gastropod gill (ctenidium), 

 which is generally behind the heart, whereas others (Nudi- 



Fig. 22. 



Sea-Hare (Telhys (Aplysia) punctata). British. 



a. labial tentacles ; b. upper tentacles or rhinophores ; c. siphonal fold 



of the mantle near the shell ; d. eye. 



branchia) have lost the true gill and have developed a different 

 type of respiratory organ. 



The Tectibranchia include the ' Bubble-shells ' (BuUidae), the 

 ' Sea-Hares ' (Aplysiidae), the ' Umbrella-shells ' (Umbraculidae), 

 the ' Pteropods ', formerly considered a distinct class, and some 

 others. In the more primitive types (e. g. Actaeon, Scaphander, 

 Bulla) the shell is well developed and untwisting of the visceral 

 nerve loop is incomplete ; in others the shell may be reduced or 

 absent. 



The Pteropods, sometimes called Sea-Butterflies, comprise two 

 groups, Pterota and Eupteropoda, 1 which are organized for 

 swimming freely in the ocean. They have a pair of tins developed 

 from the sides of the foot. The Eupteropoda are provided with 

 small glassy shells ; the Pterota are naked. Pteropods may 

 constitute an important part of the food of the Baleen Whales. 



1 The union of these two groups under a single name, though convenient 

 for several reasons, is not justifiable systematically. The Eupteropoda are 

 more correctly classified with the Bulla-Yike forms and the Pterota with 

 Aplysia and its allies. 



