n8 



THE GASTROPODA 



the osphradium is a filiform epithelial ridge, overlying a nerve or 

 ganglionated cord. Then, as a result of specialisation and multi- 

 plication of its surface, the two sides of the ridge become garnished 

 with pectinations, so that the organ acquires a deceptive resemblance 

 to a branchia. This condition may be seen in the more specialised 

 Taenioglossa, such as Natica, Cerithium, and the Strombidae, in which 

 the pectinations themselves are arborescent ; in Ct/pw?a, in which the 



FIG. 99. 



A male Hemifusus tuba, removed from its shell and with the pallia! cavity opened. I 

 anus ; II, hypobranchial gland ; III, spermiduct ; IV, renal pore ; V, heart, in the opened 

 pericardium ; VI, testis ; VII, liver ; VIII, oesophagus ; IX, columellar muscle ; X, the spernii- 

 duct cut through, with the mantle ; XI, penis ; XII, foot ; XIII, proboscis ; XIV, head ; XV, 

 pallial siphon ; XVI, osphradium ; XVII, ctenidium. (After Souleyet.) 



organ is trifid; and in the Rachiglossa (Fig. 99, XVI) and Toxiglossa. 

 In the Euthyneura the osphradium is a simple epithelial projection of 

 circular or elongated shape, lying above an osphradial ganglion, into 

 which it is sometimes invaginated viz. in certain basommatophorous 

 Pulmonates and the imagination is bifurcated in Limnaea. In the 

 last-named genus the osphradium is situated in the pallial cavity, to 

 the left of the ctenidium ; in the Basommatophora it is close to the 

 pneumostome ; in Siph&naria (in which the lung is filled with water) 



