SNAILS AND SLUGS 317 



which is traversed by a fleshy cord, continuous almost like a 

 tail with the rounded end of the visceral hump. This is also 

 connected with either side of the body-chamber by means of a 

 broad shell-muscle. The shell consists of an external membrane 

 exhibiting brown and white marking, a white porcelain-like layer, 

 and an internal layer possessing a beautiful pearly lustre, the 

 appearance of which has suggested the ordinary name of the 

 animal. 



The foot is not constituted by long tentacles or arms as in 

 the Cuttle- Fishes, &c., but consists of a number of lobes upon 

 which are borne a large number of slender adhesive tentacles, 

 the tips of which can be drawn back into sheaths. A funnel 

 is present as before, but instead of being a complete tube it is 

 made up of two halves which are rolled upon each other. Within 

 the mantle-cavity, which occupies the same relative position as 

 in a Cuttle- Fish, there are four instead of two plume-like gills, and 

 in correspondence with this the heart has four auricles, one for 

 receiving the purified blood from each gill, and there are four 

 instead of two kidneys. The eye is of extremely simple structure, 

 and has been compared to a pin-hole camera, consisting as it 

 does of a deep cup, which would be closed externally were it not 

 for the presence of an extremely small rounded aperture like a 

 pin-hole. 



CLASS 2. SNAILS AND SLUGS (GASTROPODA) 



The Ormer already described (pp. 307-3 1 1 ) belongs to this 

 class, that includes a very large number of species, of which the 

 vast majority are distinguished by the presence of a head bearing 

 tentacles, a flattened creeping foot, and a shell which consists of 

 only one valve or piece, and is therefore said to be univalve. 

 The class is split up into smaller divisions as follows: 



Sub-class i. Streptoneura (Prosobranchia). 



Order (i). Comb-gilled Snails (Ctenobranchia). 

 Order (2). Shield-gilled Snails (Aspidobranchia). 



Sub-class 2. Euthyneura. 



Order (i). Hind-gilled Snails (Opisthobranchia). 

 Order (2). Lung Snails and Slugs (Pulmonata). 



