136 DESCRIPTIONS OF PREPARATIONS. 



viscera from their muscular envelope. Its nerve in Anodonta is derived 

 from the cerebro-pedal connective (infra}. It is not seen here. 



A second connective passes backwards (downwards here) to the visceral 

 ganglion, which lies upon the inferior surface of the posterior adductor 

 muscle. It passes, first, between the fibres of the anterior retractor and of the 

 protractor pedis muscles ; and secondly, after skirting the inner edge of the 

 orifice of the reproductive gland, through the glandular portion of the 

 nephridium externally to the common tendon of the posterior retractor pedis 

 muscles. The corresponding connective of the other ganglion, i. e. that 

 of the right side, comes into view in front of as well as behind this last 

 mentioned tendon. A slip of blue paper has been passed under both 

 cords just before they enter their respective halves of the ganglion. The 

 visceral ganglion is really paired, but its two parts are closely united 

 in the median line. It gives off posteriorly to the right and left a stout 

 pallial nerve which skirts the mantle. Similar nerves are given off by 

 the cerebro-pleural ganglia, but are not seen in this preparation. Both 

 sets of nerves unite in a circumpallial plexus with ganglia here and there. 

 Other nerves (not visible) pass to the anus which lies in the median 

 line behind the posterior adductor muscle, and to this muscle itself. 

 Anteriorly the ganglion gives off two stout nerves right and left to the 

 gills. These nerves are beset in reality with ganglion cells, and are in 

 relation with a modified epithelium. The whole represents the osphradium 

 or olfactory apparatus of Spengel. There are no ganglia in a Lamelli- 

 branch corresponding to the buccal ganglia of the Snail ; but ganglia 

 may be developed along the course of the pallial nerves, and on the 

 siphonal nerves in those genera where siphons are well developed. 



The lamellae of the glandular portion of the nephridium, the inter- 

 lamellar space of the outer left gill, its interlamellar and interfilamentar 

 junctions, and a large bloodvessel running between the outer and inner 

 gills at their base, are all well seen (see ante, pp. 130-31). 



The ova must be extruded from the generative organ in part by 

 the contraction of the foot compressing the visceral mass. They escape 

 from the generative orifice where the inner lamella of the inner gill is 

 attached anteriorly to the visceral mass, and thence pass on probably as 

 follows. The free portion of the inner gill lamella is converted into a 

 canal by the apposition of the visceral mass to its edge. Behind the foot 

 the ova pass between the united inner lamellae of the inner gills of opposite 

 sides below and the organ of Bojanus above, into the cloaca. This space 

 is small in the Unionacea relatively to their ovaries : it must fill rapidly 

 with ova under pressure, and the shell being closed, there is no other 

 path for them to take but the one which leads into the interlamellar 

 space of the outer gill. This space is open to the cloaca behind the limits 

 marked by the osphradia. Spermatozoa are sometimes found free in 



