of the Fishery Board for Scotland. 



249 



organ between the posterior adductor and the foot), when not distended 

 with corpusculated fluid, is a small ridge of flabby tissue. The follicles 

 are racemose outgrowths of the minor genital canals. The follicles and 

 minor canals undergo coincident development, and may be observed pushing 

 their way in the mantle until they reach nearly to the pallial muscle. 

 With a little experience it is possible to detect the sex of the specimen 

 examined by sight, the darker orange tinge, and the less distinctly botry- 

 oidal arrangement of the female, as contrasted with the male glands, being 

 usually marked. The colour may be almost absent in comparatively 

 young forms of both sexes. The genital ducts lie mostly towards the 

 outer side of the mantle. In favourable specimens their ciliary action 

 may be seen under the microscope. Where the branches enter the fol- 

 licles they may have ciliated columnar epithelium on one side, and 

 germinal epithelium on the other (PI. XIV. fig. 1, ce, ge). The well-defined 

 canals have cilia arranged in conspicuous lines or ridges. In the large 

 canals the longitudinal ridges, formed by the infolding of the walls, may 

 project for a considerable distance into their lumen. This feature is best 

 seen in a section of the genital tube — that is, one of the two common ducts 

 which pass to the outside (PI. XIII. fig. 4). The ridges and sulci are 

 bounded by columnar epithelium, bearing long, powerful cilia. Muscular 

 fibrilke are present, forming a circular sheath, which becomes most 

 apparent at the termination of the tube, leading one to suppose that the 

 tube can be closed when there is occasion. Its orifice is often distinctly 

 lipped. Behind the orifice a cavity is formed by the bulging of the tube 

 (PI. XIII. fig. 3, b). When a piece of mature generative tissue is punctured, 

 much dense milky fluid accompanies the exuded ova or spermatozoa, that 

 with the former being usually of a much redder tinge than that with the latter. 

 The colour is quickly removed from a piece of similar tissue by methylated 

 spirit. As has been pointed out by Sabatier,* the duct of the organ of 

 Bojanus (the so-called kidney) is in close proximity to the genital duct. 

 The orifice of the former is much smaller than, and opens a short distance 

 behind the orifice of, the latter (PI. XIII. fig. 3, kt.) 



In a section of the mantle of a large adult female, fully ripe, the 

 ovarian follicles are found in a closely-packed array, with but little areolar 

 tissue visible. The ova are so numerous in them as to assume, by mutual 

 compression, the characteristic polygonal and pedunculated condition seen 

 in other mollusks — for example, in the American oyster, f and European 

 oyster. J The ova rise from the germinal epithelium of the follicle, which 

 is very delicate, and their area of attachment thereto is of considerable 

 width when development is proceeding. The nucleus and nucleolus 

 appear very early, and are large and distinct throughout intra-ovarian 

 growth (PI. XIII. fig. 2.) The vitellus is granular at an early stage, and 

 becomes gradually more so till the egg is fully ripe. A hyaline coat, pro- 

 bably albuminous, is seen to invest later intra-ovarian ova. If a section 

 be made of the mantle of a female an inch and a half in length, taken in 

 July, when the follicles are nearly empty, the few ova remaining are 

 found to be mostly oval or spherical (PI. XIII. fig. 2). In many regions 

 the germinal epithelium is seen giving rise to incipient ova, one layer, but 

 often two or more layers deep. In some of the canals granular matter is 

 found surrounding the ova. 



The epithelium of the inner (pallial) side of the mantle is richly ciliated, 

 the cilia helping to cause the currents which pass along the pallial chamber. 

 The columnar epithelium, bounding both the outer and inner sides of the 



* An. Sci. Nat., ser. vi. torn. v. pi. ii. fig. 5. 



t Brooks, Report of the Commissioners of Fisheries'of Maryland, 1880, pi. ix. 

 X Hoek. Versing Oesteronderzockingen, Leiden, 1883-84, pi. iv. fie - . 28. 



2 I 



