236 



THE SEA SHORE 



rhoidce, characterised by a pair of tentacles on the dorsal side of the 

 head and by the foot being either very narrow or absent, the latter 

 feature denoting that the animals are not adapted for creeping 

 on the bottom. In fact, several of the species of this family swim 

 freely by means of flattened tails. 



The Tectibranchs are similar in general structure, but are very 

 different in appearance, inasmuch as the gills, so prominent in the 

 last division, are here covered by the mantle, or by the shell, which 

 is often well developed. The latter is very variable in form, being 

 of a globular, twisted, spiral, or other shape, but is sometimes 

 absent in the adult. In fig. 163 we give a few examples of the shells 

 of British species ; and one (Bulla liydatis) is shown on Plate V. 



We now pass on to the largest and last order of gasteropods 

 the Prosobranchiata so called because the gills are situated in 

 front of the heart. This group is an important one to the sea-side 



FIG. 163. SHELLS OF TECTIBRANCHS 



naturalist, since it contains nearly all the univalve molluscs that 

 are common between the tide-marks of our shores, as well as some 

 abundant species that are protected by a shell of several distinct 

 parts. In nearly all of them the abdomen is well developed, and 

 the shell is sufficiently large to cover the whole animal when the 

 latter is retracted ; and the gills, which are either pectinated (comb- 

 shaped) or plumed, are lodged in the chamber formed over the head 

 of the animal by the mantle. 



The order is often divided into two sections the Holostomata 

 or Sea Snails, in which the margin of the aperture of the shell is 

 entire, and the Siphonostomata, in which the margin of the mantle 

 is prolonged into a siphon by which water passes into the gill 

 chamber. This division does not seem to be very satisfactory, as 

 the sections are not separated by very prominent natural character- 

 istics, but it becomes convenient on account of the great extent of 

 the order. 



