118 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 Cypraca, in which the 
Xt 
XI 
Fie. 99. 
A male Hemifusus tuba, removed from its shell and with the pallial cavity opened. I 
anus ; II, hypobranchial gland; III, spermiduet; IV, renal pore; V, lieart, in the opened 
pericardium ; VI, testis; VII, liver; VIII, oesophagus ; IX, columellar inuscle ; X, the spermi- 
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 invagination is bifureated in Limnaca. 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 S¢phonaria (in which the lung is filled with water) 
