THE GASTROPODA 67 



I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. 



The three external divisions of the body, head, foot, and mantle 

 are well defined. The head is well developed, and forms a more or 

 less cylindrical mass, but is sometimes flattened. At its anterior 

 extremity is the mouth, and dorsally it bears one or two pairs of 

 tentacles. There is one pair of tentacles in the Streptoneura 

 (Fig. 44), the " Thecosomata " (Fig. 63), in Phyllirhoe (Fig. 161), 

 Thecacera, the Proctonotidae, the Elysiidae (Fig. 170), the basom- 

 matophorous Pulmonates, and Janella (Fig. 178). There are two 

 pairs in the majority of Opisthobranchs (Fig. 154) and in the 

 stylommatophorous Pulmonates (Fig. 172). The tentacles either 

 are or bear sensory organs ; they are contractile, and in the Stylom- 

 matophora invaginable. The right tentacle of both sexes bears an 

 appendage in Bathysciadium (Fig. 126), certain Trochidae (Fig. 130) r 

 and Calyptraea. The form of the tentacles varies greatly in different 

 groups. Sometimes they are atrophied, and they may even dis- 

 appear without leaving a trace, as in Olivella, Homalogyra, certain 

 species of Terebra, Pterutrachea (Fig. 143), Limapontia, and Pseudo- 

 vermis (Fig. 169). In the majority of the Bullidae the two pairs 

 of tentacles are enlarged and transformed into a quadrangular shield 

 (Fig. 148), the four corners of which correspond to the tips of the 

 four tentacles. The single pair, much reduced in certain Basom- 

 matophora (Amphibolidae, Otinidae, Fig. 173, Siphonariidae), 

 similarly gives rise to the appearance of a flattened disc on the top 

 of the head. The anterior pair of tentacles in the Pleurobranchidae 

 (Fig. 157) and in various Nudibranchs (Tritoniidae, Fig. 83, Den- 

 dronotidae, Tethyidae, etc.) is transformed into a more or lesa 

 well -developed frontal veil. Finally, the tentacles are flattened 

 (Xnnca) ; split (Pyramidellidae, Fig. 137, Solarium, the posterior 

 pair in many Opisthobranchs) ; bifurcate (Janthina, certain Elysio- 

 morpha) ; or multifid (the posterior pair in many Nudibranchs^ 

 Dcndr&notus, Ancula, Fig. 163). In some species, on either side of 

 the buccal orifice, there is another pair of appendages of greater or 

 less length, known as the labial palps. These are found in Trochus 

 infundibulum, Ampullaria, Jeffreysia, Choristes, among the Strepto- 

 neura ; and among the Euthyneura in sundry Pulmonates (Glandina, 

 Limnaea, in which they form a sort of buccal veil, Fig. 107), and 

 in Tectibranchs. In addition to the above-mentioned cephalic 

 appendages of the adult, the following structures should be noted : 

 the cephalic or frontal lobes, situated between the two tentacles 

 and consisting of projections of various shape, in many Rhipido- 

 glossa and in Fossarus ; the dorsal median crest in Olivella and 

 Janus ; and finally, the pseudopallium, an expansion of the 

 cephalic integument surrounding the whole shell, with the excep- 



