GASTKOPODA. 



23 



Section Euthyneuea. 



The Gastropods belonging to this sub-class have the visceral Cases 

 nerve-loop straight and not twisted as in the Steeptoneura. All 9i-l35. 

 the Euthyneuea 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 Euthyneuea may be divided into two 

 orders, Opistholranchia and Pulmonata. 



Order 1. — Opisthobeanchia. 



All the Molluscs of this order are marine, some {Tectibrancliid) Cases 

 breathing by means of the ordinary Gastropod ctenidium, which is 

 generally behind the heart, whereas others {Nudihranchia) have 

 developed a different type of respiratory organs. 



The OpkiliolrancMa include the '' Pteropods " forlnerly con- 

 sidered as a distinct class, the " Bubble-shells " {Bidlidce), the " Sea- 

 Hares" {Aplysiidce), the "Umbrella-shells" {Umhraculidce), the 

 Nudibranchs 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 (CavoUnia tridentata). Case 96. 

 a. Shell and animal, h. 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 pous, foot. 



