2 22 Records of the Indian Museum. [Vol. XXII, 



well known that the syphon is exhalent and not inhalent as he 

 supposed (I.e., p. 4). 



The middle portion of the mantle is, as has already been 

 mentioned, thin, and by carefully cutting through along this line 

 and everting the two sides we are able to see the various structures 

 contained within the cavity. The left side of the mantle is the 

 same in both sexes. ■ Commencing in the middle line in front and 

 running backwards and to the left is the line of attachment of the 

 gill. This is of the pectinate type and comprises roughly some 

 three hundred filaments. Each filament or plate is of an elongate 

 triangular shape, with a narrow base of attachment and tapering 

 towards its extremity'. The basal attached portion is to the left 

 and the free margin of the gill lies towards the right side. Each 

 gill-filament is covered with ciliated epithelium. Immediately on 

 the right and, owing to the oblique course of the gill, a little 

 posterior to it, is a raised, brownish-coloured ridge, the hypo- 

 branchial gland, which extends upwards along the whole length of 

 the gill-base. It is covered with a tall columnar epithelium and 

 lying immediately above it is a large blood sinus, the afferent 

 gill-vein, which collects blood from the viscera and conducts it to 

 the gills, where it is aerated. To the left and in front of the gill 

 and extending backwards from just behind the edge of the 

 mantle to a point about one-third the length of the gill is a raised 

 narrow ridge of a whitish yellow colour ; this is the osphradium, 

 and from its upper end a narrow white line is continued upwards, 

 parallel to the gill-base as far as the extreme apex of the mantle 

 cavity. As already mentioned, the mantle cavity is divided into 

 right and left halves by the branchial fold. Water entering the 

 mantle cavity passes into the left of this fold, and having 

 traversed the branchial chamber passes out again down the right 

 side and through the syphon. From the position of the osphra- 

 dium, which commences a little to the left of the middle line and 

 runs outwards and backwards on the left and in front of the gill, 

 it is evident that all the water entering the cavity ijiust first pass 

 over the osphradium before it can reach the gill- filaments. The 

 osphradium forms a very sensitive sensory organ. It is covered 

 with ciliated epithelium and on its gill-surface it bears a number 

 of small microscopic pits, which are also lined by columnar epithe- 

 lium. These pits are difficult to see in the natural §tate, but in 

 some cases indications of them can be made out owing to the mouths 

 being faintly outlined with yellow pigment. The organ is supplied 

 by a special nerve or series of nerves, the osphradial nerves, which 

 arise from the anastomosis between the mantle nerve, coming 

 from the left pleural ganglion and the first gill-nerve arising from 

 the supraintestinal ganglion of the right visceral nerve (for a 

 detailed account of this organ in Vivipara vivipara see Bernard, 

 1890, pp. 244-250). 



On the right of the mantle chamber are the rectum and ureter 

 and, in the female, the uterus also. These pass down close 

 together on the right wall of the mantle chamber and open close 



