196 Description of the Plates. 



s. Secretory or glandular portion of the organ of Bojanus, 

 reaching- from the level of the anterior end of the peri- 

 cardial space to the under surface of the posterior adductor. 

 It opens into the pericardium by a canal along which a 

 bristle has been drawn as passing. It is seen to be covered 

 by the excretory half of the bisacculate organ for a space 

 corresponding with the under surface of the pericardium. 

 With this sac it is seen to communicate by a very fine 

 orifice at q. Posteriorly to this point it is prolonged into 

 a convolutionary mass, roughly drawn here in section as 

 sub-triangular, in relation with the posterior adductor and 

 the tendon of the posterior retractor. These glandular 

 lamellar sacs communicate freely with each other, as do 

 also the excretory sacs in this species. 



t. Orifice leading to ramifications of the duct of the generative 

 gland. This orifice is in the Anodon, though not in the 

 Unios, concealed by the attachment of the inner gill-lamina 

 to the visceral mass. See V. Baer, MeckeFs Archiv., 1830, 

 p. 318. 



From this semi-diagrammatic figure, the course which the ova 

 take in passing from the generative orifice, t, to the external 

 gill-cavity, where they meet with the spermatozoa inhaled 

 with the water they breathe, and where they go through 

 certain stages of development, may be understood. The ova 

 are extruded from the orifice specified by the contraction of 

 the several muscles, g, h, Ji, and f; the shell valves being 

 appressed by the adductors, d and e. When they pass out 

 from this orifice, they pass along a canal, which for the 

 first part of its course corresponds in direction with the 

 nerve cord seen passing through the organ of Bojanus to the 

 parieto-splanchnic ganglion, I. This first part of the canal 

 is divided into three portions. The first of these is formed 

 by the attachment of the innermost gill-lamina to the vis- 

 ceral mass, and into it the orifice t opens. The second is 

 under ordinary circumstances only a demi-canal, but is 

 completed during the act of the extrusion of the ova, by 

 the close apposition of the inner gill-lamina to the side of 

 the visceral mass, which under these circumstances becomes 

 more globular superiorly than when quiescent. The third 



