166 THE GASTROPODA 



pore. FAMILY 1 5. HARPIDAE, Chenu. Foot very great ; without oper- 

 culum ; shell with a short spire and longitudinal ribs ; siphon long. 

 Genus Harpa, Lamarck. 



TRIBE 2. TOXIGLOSSA. 



Stenoglossa without jaws, and with a radular formula of 1.0.1 ; a 

 * l poison gland " present, whose duct traverses the nerve-collar. 



FAMILY 1. PLEUROTOMATIDAE, Loven. Shell fusiform with an elongated 

 spire ; the margin of the shell and mantle notched ; siphon rather long ; 

 eyes situated on the sides of the tentacles. Genera Clavatula, Lamarck ; 

 operculum piriform with a lateral nucleus ; eyes near the extremities of 

 the tentacles. Pleurotoma, Lamarck ; operculum oval with nucleus near 

 the summit ; eyes near the bases of the tentacles. Mangilia, Risso ; no 

 operculum; eyes half-way up the tentacles. Beta, Gray. Pusionella, 

 Gray. Pontiotliauma, Smith. FAMILY 2. TEREBRIDAE, Adams. Shell 

 turriculated, with numerous whorls ; aperture and operculum oval ; foot 

 small ; eyes at the summits of the tentacles ; siphon long. Genus 

 Terebra, Adanson. FAMILY 3. CONIDAE, Gray. Shell conical, with a 

 very short spire and a narrow aperture with parallel borders ; eyes 

 borne near the middle of the external sides of the tentacles ; an ungui- 

 form operculum. Genus Conns, Linnaeus (Fig. 144). 



SUB-CLASS II. EUTHYNEURA, Spengel 

 ( = Platymalakia, von Jhering = Androgyna, Morch). 



These are hermaphrodite Gastropoda, whose radula is generally 

 composed of uniform teeth on each side of the median tooth 

 (Fig. 1 45). The head in most cases bears two pairs of tentacles ; 



it is only in Lophocercus, 

 the Elysiomorpha, 

 Hedyle milaschewitchi, 

 and the Janellidae that 

 there is a single pair. 

 The Euthyneura are 

 Fl - 145 - specially characterised 



Acera bullata, a single row of teetli of the radula; fonmila : v f i rlpf^rcinn nf tliPi'r 

 .!.. (From Lankester, after Loven.) D y tj 



organisation when adult; 



this detorsion is particularly well manifested in the visceral com- 

 missure, which is no longer twisted, except in some archaic forms 

 of Tectibranchs (Actaeon, Fig. 57) and Pulmonates (Chilina), and 

 shows a tendency to the concentration of all its elements round 

 the oesophagus (Fig. 146). To such a degree is this concentration 

 carried that, with the exception of the majority of the Bullomorpha 

 and of dplysia (Fig. 95), the whole central nervous system is 

 aggregated in the cephalic region (Fig. 97), sometimes on the dorsal 

 side, as in the Pleurobranchidae and Nudibranchia (Fig. 159), 

 sometimes on the ventral side as in the Thecosomata (Fig. 60, n.s). 

 The pedal centres are frequently united by a second " parapedal " 



