UNIVALVES. 51 



may be said as a whole to breathe by gills, by lungs, and by the 

 outer skin. 



The Pulmonates, those that breathe by lungs, are divided into 

 two sub-orders the Basommatophora, in which the eyes are 

 generally at the base of the tentacles, and the Stylommatophora, in 

 which the eyes are at the tip of the tentacles. We have already 

 mentioned the Prosobranchiates, with their two sub-orders of 

 Diotocardia and Monotocardia, the first divided into Docoglossa 

 and Rhipidoglossa, the second into Pteno , Taenio , Gymno , 

 Rachi , and Toxoglossa ; and the only gastropodous order left is 

 the Opisthobranchiates, one sub-order of which, Ascoglossa, has 

 also been mentioned, the others being the Tectibranchs, the 

 Nudibranchs, and the Pteropods. 



The Pteropods used to have an order all to themselves, but are 

 now claimed to be gastropods adapted to a pelagic life by the 

 modification of their foot into fins. Only three of their species are 

 recognised as British. The Nudibranchs are the sea-slugs, gorgeous 

 in protective colouring, that have no shell in the adult state and 

 breathe either by the skin or by the organs developed on the back, 

 which give them their characteristic appearance. The Tectibranchs 

 have a shell more or less rudimentary and more or less enveloped in 

 the foot or mantle, and their breathing is done by a single gill, also 

 more or less covered by the mantle. 



Between the Pelecypods and the Gastropods there are now 

 placed the Scaphopods, which, unlike them, are not divided into 

 orders, and consist of but one family, the Dentaliidae. In our seas 

 but three genera are represented, two of which are very rare. 

 Dentalium, the common one, has a shell like a model elephant's 

 tusk, some ten times as long as its diameter. The shell of Siphono- 

 dentalium is also tusk-like, but not more than five diameters in 

 length, and further distinguishable by the four notches at its base. 

 Cadulus is of a different shape, as can be seen by the drawing, and 

 neither of them is likely to be mistaken for Dentalium. 



In the Scaphopods the mantle has two folds to begin with, and 

 these become united so as to deposit the cylindrical shell, which is 

 open at both ends, the larger being the anterior, from which the 

 long foot with its three terminal lobes is protruded, on which the 

 animal creeps and by which it burrows in the sand. The head is 

 small and cylindrical, the mouth surrounded by tentacles and in it 

 a simple radula. The ventral side is the convex side. There is a 

 blood circulation, but no heart ; there are plenty of nerves, and, at 

 least, an organ of hearing ; and the sexes are separate. Simple as 

 the elephant's tooth may look, it is highly organised enough to 

 be placed by some near the Cephalopods, with the Gastropods 

 below it. 



The shell of a gastropod may be divided into body-whorl and 

 spire, as in the annexed diagram. The spire may consist of any 

 number of whorls, increasing in size from the apex downwards. At 

 the apex the shell began, and each whorl in turn was the body-whorl. 

 The suture is the line of junction between the whorls, and it descends 

 all the way from the apex to the mouth. As the shell grew the inner 

 side of its whorls formed the columella or central pillar, around 

 which they are arranged like steps in a spiral staircase. The mouth 



