400 THE MUSSEL chap. 



passage unites with its fellow behind the visceral mass, see Fig. 95c). 

 The inner canal also communicates with the branchial chamber through 

 the slit between the visceral mass and the middle part of the lamella of 

 the inner gill. Cut open the canals carefully, and examine their 

 relations. 



6. Turn up the gills, and insert the scissors in the slit just mentioned ; 

 cut forwards through the inner lamella of the inner gill for a short dis- 

 tance. The renal aperture or nephridiopore can then be seen opening 

 into the suprabranchial passage, and just below and in front of it is the 

 genital aperture. Then note the paired reno-pericardial aperture {ne- 

 phrostome) in the anterior end of the pericardium, just below the point 

 at which the rectum enters it : this can be more easily seen if the rectum 

 is cut through and raised up. 



7. Blow through the nephridiopore, and note the thin-walled sac-like 

 bladder into which it opens : this lies just below the pericardium, and 

 communicates with its fellow anteriorly. Just beneath and internal 

 to the bladder is the glandular portion of the kidney, which is dark in 

 colour and extends further back than the bladder, beyond which it forms 

 a large mass just in front of the posterior adductor muscle : it communi- 

 cates with the hinder end of the bladder. Sketch. 



III. I. Remove both gills carefully, cutting along their bases; stain 

 and mount a small piece of one lamella. Note that the lamella is made 

 up of close-set vertical bars or gill-filaments, connected by numerous 

 tranverse inter-filamentar junctions ; the bars are covered with ciliated 

 epithelial cells, and each is strengthened by pairs of small chitinous 

 rods. The whole gill is traversed by blood vessels. Sketch. 



2. A small portion of a gill should be preserved, stained, inbedded, 

 and ;cut into sections (p. 136) passing transversely through the gill- 

 filaments. Compare with Fig. 94, and sketch. 



3. Mount in salt-solution a small piece of fresh gill, and also of the 

 mantle, and observe the movements of the cilia. 



IV. If your specimen is a female, and contains ova or larvse in the 

 outer gill, examine some under the microscope. The ova are provided 

 with a canal or inicropyle perforating the vitelline membrane, for the 

 entrance of the sperms. Note the form of the larvte or glochidia 



(Fig- 97). 



V. The nervous system, consist of three pairs of small orange-coloured 

 ganglia, with connectives between them. 



I . Cut &way the left labial palps very carefully and look for the left 



