GASTEROPODA 495 



terminal sacs in the cerata and when the animal is irritated they are 

 ejected and everted. This is a unique example of the use in defence 

 by one animal of the offensive weapons of another. The cerata are 

 often brilliantly coloured and experiments with fish show that sea 

 slugs are avoided on account of their "warning" patterns. 



Hermaea is another nudibranch with similar cerata, which have not, 

 however, openings to the exterior. The animal feeds on green algae 

 (Siphonales). The radula, in each row of which there is only a single 

 sharp tooth, forms a saw by which the cell wall of the alga is opened. 

 Then by dilatation of the buccal cavity the fluid protoplasm is sucked 

 out. 



Doris (Fig. 364 B), the sea lemon, a short flattened nudibranch, 

 sluggish in movement, which feeds on incrusting organisms like 

 sponges. There is a tough mantle, which is usually pigmented and 

 often resembles the feeding ground, and is reinforced by calcareous 

 spicules. Anteriorly there is a single pair of short tentacles and 

 posteriorly a median anus surrounded by a tuft of accessory gills. The 

 nervous system is centralized round the oesophagus, and the generative 

 aperture occurring on the right side is the only external organ which 

 is asymmetrical. 



Order PULMONATA 

 Hermaphrodite gasteropods, most of which exhibit torsion and have 

 a shell (but no operculum), but which have a symmetrical nervous 

 system, the symmetry being due to the shortening of the visceral 

 connectives and the concentration of the ganglia in the circum- 

 oesophageal mass; with a mantle cavity which has become a lung, 

 without a ctenidium, but with a vascular roof and a small aperture 

 (pneumostome) ; with a single kidney; without a larva, development 

 being direct from an egg richly supplied with albumen. 



The Pulmonata are thus classified : 



Basommatophora. Pulmonata with eyes at the base of the posterior 

 tentacles. Limnaea, Planorbis. 



Stylommatophora. Pulmonata with eyes at the tip of the posterior 

 tentacles. Helix^ Avion, Testacella. 



A few pulmonates are marine but these are all shore forms and 

 breathe air. The group, like the Opisthobranchiata, must have been 

 derived from the Streptoneura Monotocardia, as they possess a single 

 kidney. While they are usually united with the Opisthobranchiata to 

 form the Euthyneura, which includes all forms in which the visceral 

 loop is untwisted, there is no real justification for the establishment 

 of the group, for the " euthyneurous " condition is one which has been 

 arrived at in two different ways, by detorsion in the Opisthobranchiata 



