vin PRACTICAL DIRECTIONS 415 



5. Pass a seeker forwards from the exhalant siphon along the supra- 

 branchial passages two longitudinal canals at the bases of the gills 

 communicating with the interlamellar spaces or water-tubes. The inner 

 canal unites with its fellow behind the visceral mass (see Fig. 103, c), 

 and 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 

 transverse inter-filamenlar jtmctions ; 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 the gill should be preserved, stained, imbedded, 

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

 filaments. Compare with Fig. 102, and sketch. 



3. Mount in salt-solution a small piece of the edge of a 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 larvae in the 

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

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

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

 (Fig. 105). Sketch. 



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

 ganglia, with connectives between them. 



