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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 Phyllirhoé (Fig. 161), 
Thecucera, 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 Buthysciadium (Fig. 126), certain Trochidae (Fig. 130), 
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, Pterotrachea (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 less 
well-developed frontal veil. Finally, the tentacles are flattened 
(Narica) ; split (Pyramidellidae, Fig. 137, Solarium, the posterior 
pair in many Opisthobranchs) ; bifurcate (Janthina, certain Elysio- 
morpha); or multifid (the posterior pair in many Nudibranchs, 
Dendronotus, 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 7vochus 
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- 
