324 



CHARACTERS OF INVERTEBRATE ANIMALS 



elusion is afforded by the life-history, for at an early period of 

 its existence, when it is a free-swimming larva, it actually does 

 possess spirally twisted visceral hump and shell. This is another 

 example of the law of recapitulation previously exemplified (see 

 p. 14). 



Sub-class 2. — EUTHYNEURA 



These are forms in which the nerve-loop of the nervous system 

 is not twisted (except in one family), but it would appear that 

 this is not, as might at first sight be supposed, a primitive feature, 

 but the result of an untwisting process. A further character is 

 the possession of two pairs of tentacles by the head. There are 

 two orders : i. Hind-gilled Snails (Opisthobranchia), with the 

 auricle of the heart behind the ventricle, and the gill in a 

 corresponding situation ; and 2. Lung Snails (Pulmonata), in 

 which the gills are entirely absent and the mantle-cavity has 

 been converted into a lung. 



Order i. — HiND-GlLLED SNAILS (Opisthobranchia) 



A very great variety of marine forms are placed in this order, 

 some snail-like in appearance, others slug-like, and others again 

 of modified shape and adapted for swimming in the open sea. 

 A distinction is drawn between species in which 

 there is typically a gill sheltered in the mantle- 

 cavity (Tectibranchs), and the Sea-Slugs (Nudi- 

 branchs), devoid of mantle and shell. 



Among the Tectibranchs the Bubble-Shells, 

 possess a thin translucent spiral 

 shell (fig. 187), overlapped by a 

 body-fold (epipodium) each side. 



Another related family is exem- 

 plified by a small white mollusc, 

 Philine aperta, very common in 

 shallow water round the British 

 coast. The shell is something 

 like that of a bubble-shell, but 

 is not visible externally, as folds of the mantle have grown 

 completely over it. In the Sea- Hare {Aplysid) the shell is still 

 further reduced, being a thin oval plate situated on the upper 



Fig. 187.— Bubble-shell [Bulla) 

 a. Head-lappets; b, right epipodium; c, shell; 

 d, mantle-lobe; e, hind-end of foot;^ shell. 



