326 CHARACTERS OF INVERTEBRATE ANIMALS 



these are not formed from the foot, but from the region of the 

 body immediately above it, and equivalent to flaps found in this 

 position in the Sea- Hare (epipodia) or to the halves of the funnel 

 in a Pearly Nautilus (see p. 317). 



Sea-Slugs (Nudibranchs). These are beautifully coloured 

 creatures with a large creeping foot. Their symmetrical form 

 is not a primitive character, but due to the untwisting process 

 just described for the Sea- Hare. Here, however, modification 

 has gone a stage further, for not only is there no shell, but 

 both mantle-cavity and the typical plume-like gill are absent. 

 The intestine opens in the middle line* on the posterior part of 

 the body. 



A common British genus is Doris, in which the absence 

 of plume-gills is made up for by the presence of a circlet of 



branched secondary 

 gills situated on the 

 upper side of the 

 body around the 

 opening of the in- 

 testine. If the ex- 

 Fig. i8 9 .-Eolis 



panded gills of a 



living specimen be touched they are immediately drawn in, being 

 sheltered in a ring -like groove when so retracted. Another 

 common genus is Eolis (fig. 189), in which the back is studded 

 with numerous slender club-like processes. 



A very interesting little Nudibranch is the free-swimming 

 Mediterranean form Phyllirhoe, which possess a transparent 

 laterally-flattened body and is devoid of foot. There are numerous 

 little phosphorescent bodies in the skin. 



Order 2. LUNG SNAILS (Pulmonata) 



The seventeen families of this order are mostly inhabitants 

 of the land or of fresh-water, and familiar examples are furnished 

 by the land-snails and slugs. The common Garden Snail (Helix 

 aspersa) may be taken as an illustrative type (fig. 190). 



The part of the body which is protruded from the shell when 

 the animal crawls is symmetrical, and its under part is made 

 up of the well-developed foot, which has a rounded front end and 

 ends in a point behind. The head is fairly distinct, and bears 



