144 



THE GASTROPODA 



xxi 



right side ; the connective passes from the right pleural to the 

 infra -intestinal ganglion, and may have the effect of bringing 



the latter ganglion between the two 

 pleural centres (Fig. 123, C, i.i.g). 

 The head of Streptoneura bears only 

 a single pair of tentacles (Fig. 125, a). 

 The radular teeth, when there is more 

 than one on either side of the median 

 tooth, are of several different kinds 

 in each transverse row (Fig. 74, C, F). 

 The heart is almost always posterior 

 to the branchia. The sub -class in- 

 cludes two orders, Aspidobranchia and 

 Pectinibranchia. 



xx 



XIX 

 XVIII 



FlO. 124. 



Trochus cinerarius, central nervous 

 system, dorsal aspect, with the anterior 

 part of the digestive tract. I, salivary 

 gland ; II, cerebral ganglion ; III, 

 cerebro-pleural connective ; IV, pleural 

 ganglion ; V, right pallial nerve ; VI, 

 pedal ganglion; VII, supra -intestinal 

 part of the visceral commissure ; VIII, 

 posterior part of the glandular oeso- 

 phagus ; IX, infra - intestinal part of 

 the visceral commissure ; X, abdominal 

 ganglion ; XI, oesophagus ; XII, radula ; 

 XIII, supra-intestinal ganglion ;, XIV, 

 osphradial ganglion and branchial 

 nerve ; XV, left pallial anastomosis or 

 dialynmiry ; XVI, glandular oesopha- 

 gus ; XVII, left pallial nerve; XVI II, 

 buccal mass ; XIX, cerebro-pedal con- 

 nective ; XX, stomato-gastric ganglion ; 

 XXI, snout. 



opening to the exterior at 

 The gonad has no accessory 



ORDER 1. Aspidobranchia. 



These are Streptoneura in which 

 the nervous system is still but little 

 concentrated (Fig. 124). The pedal 

 centres have the form of long gan- 

 glionated cords, to the anterior end 

 of which the pleural centres are 

 attached : the cerebral ganglia are 

 widely separated from one another, 

 and are united by a long commissure 

 lying in front of the buccal mass and 

 the salivary glands (Fig. 127, c.c). 

 An infra - oesophageal or "labial" 

 cerebral commissure is present. The 

 osphradium is but little specialised, 

 and is situated on the branchial nerve. 

 The otocyst contains numerous oto- 

 conia. The eye is open (Fig. 100), or 

 if closed has a very small pellucida. 

 The central teeth of the radula are 

 multiplied. Ctenidia are almost 

 always present; they are bipectinate 

 and free at their distal ends (Fig. 

 81, d). As a rule, the Aspidobranchs 

 exhibit well-marked traces of the 

 original bilateral symmetry, having 

 two auricles to the heart and two 

 kidneys (Fig. 127), the last named 



the end of short papillae (Fig. 88, /). 



organs and discharges its products into 



