GASTROPODA. 
23 
Section Euthyneura. 
The Gastropods belonging to this sub-class have the visceral Cases 
nerve-loop straight and not twisted as in the Streptoneura. All 9 *~ u,) - 
the Euthykeura are hermaphrodite, and their radula is generally 
composed of numerous similar denticles on each side of a median 
tooth. Scarcely any of these forms are provided with an operculum 
in the adult state. The Euthykeura may be divided into two 
orders, Opisthobmnchia and Pidmonata. 
Order 1. — Opisthobranchia. 
All the Molluscs of this order are marine, some (Tectibranchia) cases 
breathing by means of the ordinary Gastropod ctenidium, which is 94-97. 
generally behind the heart, whereas others {Nudibranchia) have 
developed a different type of respiratory organs. 
The Opisthobranchia include the " Pteropods " formerly con- 
sidered as a distinct class, the " Bubble-shells " (Bidlidce), the " Sea- 
Hares" (Aplysiidce), the "Umbrella-shells" (Umbraculidce), the 
Niidibranchs and some others. 
The Pteropods* are sometimes called Sea-butterflies, and are 
organized for swimming freely in the ocean. They have a pair of 
Fig. 16. 
Shell-bearing Pteropod (Cavolina tridentatd). Case 96. 
a. Shell and animal, b. Side view of shell, c. Dorsal view of shell. 
fins developed from the sides of the mouth or neck, which perform a 
flapping movement during progression. Some Pteropods (Thecoso- 
* From the Greek : pteron, wing, and pons, foot. 
