vra PRACTICAL DIRECTIONS 415 



with small meshes or ostia, separated by horizontal and 

 vertical bars. Cut away a piece of the outer lamella, noting 

 that the two lamella? of each gill are united at intervals by 

 inierlamellar junctions. 



4. Make out the mode of attachment of the gills (see 

 p. 404, and Fig. 103). Sketch your dissection with the 

 gill-laminae in their natural position. 



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-lamina. Cut open the canals 

 carefully, and examine their relations. 



6. Turn up the gill-laminae, and insert the scissors in 

 the slit just mentioned ; cut forwards through the inner 

 lamella of the inner lamina for a short distance. The renal 

 aperture or nephridiopore can then be seen opening into the 

 supra-branchial passage, and just below and in front of it 

 is the genital aperture. Then note the paired reno-pericardial 

 aperture (nephrostome) 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 communicates with the hinder end of the bladder. 

 Sketch. 



III. i. Remove, both gill-laminae 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- 

 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 the gill should be preserved, stained, 

 embedded, and cut into sections (see p. 136) passing trans- 



