OP1STHOBRANCHIATA. 



403 



all approximated to the oesophagus (except in the Bulloids and 

 in Aplysia). In the Bulloids it shows traces of the characteristic 

 torsion of the Streptoneura ; and in one form, Actaeon, it is actually 

 streptoneurous (Fig. 318). Hermaphroditism is universal and the 

 generative system is complicated by the presence of a number of 

 accessory structures (albu- 

 men and mucous glands, 

 spermathecae, etc.). 



The otocysts generally 

 contain otoconia. Fur- 

 ther, there is in this order 

 a tendency to the reduc- 

 tion of the pallial cavity, 

 the shell, and the oper- 

 culum. The operculum 

 is absent in almost all 

 forms; the shell is com- 

 pletely absent in Kudi- 

 branchs, and it tends to 

 disappear in Pulmonates 

 and Tectibranchs. The 

 mantle-cavity is absent in 

 Nudibranchs, and is but slightly marked off in most Tectibranchs; 

 and in cases in which it is present its reduction is indicated by 

 the fact that the genital ducts generally open outside it. 



As explained on p. 385, the Euthyneura may be looked upon as streptoneurous 

 forms which have undergone partial detorsion, the result of which is that the 

 visceral commissure is generally untwisted, and the mantle-cavity and external 

 'openings are, as a rule, shifted off the anterior face of the visceral sac to the 

 right side, and often displaced far backwards. 



Sub-order 1. Opisthobranchiata. 



Marine Euthyneura with aquatic respiration. The gills are 

 generally behind the heart. The pallial-cavity when it exists is 

 widely open. 



The mantle-cavity never contains more than one ctenidium, placed 

 on the right side of the body. Often there is no ctenidium 

 (Nudibranchs). 



Tribe 1. TECTIBEANCHIATA, 



Opisthobranchs provided with a mantle, and a shell (except in JKuncina, 

 Pleurobranchaea, the Pneumatodermatidae, Clionopsidae, and Clionidae), with 

 a ctenidium (except in the three above-named families), and with an osphradium. 



FIG. 316. Nervous system of Limnaea (after L. Du- 

 thiers). Cg cerebral, Pg pedal, Pig pleural, Ag 

 abdominal ganglion ; osphradium. 



