BULLAS AND SEA-BUTTERFLIES. 



639 



SUB-CLASS B.— EUTHYNEURA. 



Gastropoda, in which the visceral nerve-loop, with one exception, ia not 

 twisted into a figure of 8, and in which the nerve ganglia are brought closer 

 together around the oesophagus. The twist in the visceral mass is not so 

 great as in the preceding sub-class, and the pallial cavity with its accompany- 

 ing organs lies on the right side of the body. The head generally bears two 

 pairs of tentacles ; the radula is usually composed of rows of uniform teeth 

 on either side of a central one ; and the sexes are united in each individual. 



The Euthyneura are divisible into two orders — Opisthobranchiata and 

 Pulmonata. 



Order I. — Opisthobranchiata. 

 In-this group the heart is placed in front of the gills. 

 divided into Tectibranchiata and Nudibranchiata. 



The order is sub- 



Sub-order 1. — Tectibranchiata. The animal possesses a shell usually 

 more or less completely concealed by the folds of the mantle and skin. For 

 convenience this sub-order is further divided into three groups — Bulloidea, 

 Aplysioidea, and Pleurobranchoidea. 



A. — Bulloidea. 

 The Bulloidea have a well-developed shell, external or internal (Fig. 18 

 A-C) ; the head ia generally devoid of tentacles, and the skin of the upper 

 part is produced into a broad disc. In many the stomach ia armed with 

 shelly plates for crushing the shells of their prey. 



Pig. 18.— TSOTIBEANCUS. 



A, Actason. 



B, Bulla. 



C, Philine. 



D, Dolabella. 



C^^3 



g. 19.— Pteropods. 



A, Spiralis. 0, Cleodora. 



B, Eur^bia. D, Limacina 



The genus Adcuin is distinctly streptoneurous and thus forms a con- 

 necting link between the Streptoneura and Euthyneura. It has an 

 ovate, spired shell, with pointed apex and a narrow aperture that widens 

 out in front. The horny operculum ia elliptical in shape. 



