MARINE MOLLUSCS 233 



be supposed that all the oxygen required could be absorbed directly 

 from the water through their soft structures, as is really the case 

 with many aquatic creatures ; and as a matter of fact some of the 

 nucleobranchs possess no gills, but others have these organs fully 

 formed. 



Passing now to the true gasteropods, we shall first consider the 

 Opisthobranchs, which are commonly known as Sea Slugs and Sea 

 Lemons. Some of these have no shell at all, and even where one 

 exists it is very rudimentary, usually very small and thin, and con- 

 cealed within the mantle. The gills are either branched and tree- 

 like, or are composed of tufts or bundles of filaments ; and, as the 

 name of the order implies, are situated towards the posterior part of 

 the body. They are also retractile, and when the animal is alarmed 

 it will conceal its gills, thus reducing its body to a shapeless, slimy 

 mass, inviting neither to sight nor to touch. 



The sea slugs are principally animal feeders, subsisting on 

 small crustaceans, other molluscs, &c.; the food being first reduced 

 by the rasping action of the teeth, and then masticated in a gizzard 

 which is provided internally with horny spines or hard, shelly 

 plates. 



It will not be necessary to enumerate all the different families 

 of this order, especially as the species are mostly to be found beyond 

 the tide-marks, and are therefore obtained only with the aid of the 

 dredge ; but we shall describe a few of the British species with a view 

 of showing the general characteristics of the animals. 



They are usually divided into two sections, those with exposed 

 or naked gills (NudibrcmcJiiata) forming the first, and those in 

 which the gills are covered either by the shell or the mantle (Tecti- 

 branchiata) comprising the second. 



In the Nudibranchs the shell exists only during the embryonic 

 stage, and the external gills are arranged on the back or along the 

 sides. The tentacles are not employed as organs of touch, but are 

 probably connected only with the sensation of smell, being provided 

 with filaments of the olfactory nerve ; the eyes are small dark- 

 coloured spots embedded in the skin behind the tentacles. Various 

 species are to be found on all rocky coasts, where they range from 

 low-water mark to a depth of fifty or sixty fathoms, but a few are 

 pelagic, living on the surface of floating sea weeds. 



It is almost impossible to identify the species of nudibranchs 

 from dead specimens, for the classification of the section is based 

 largely on the arrangement of the gills, which are almost always 



