DIGESTIVE AND EEPEODUCTIVE SYSTEMS 97 



The ovary, or testis, is very large, and, when the sexual 

 products are ripe, fiUs up the greater part of the visceral 

 mass between the foot and the Mdneys. The ducts converge 

 on each side to the genital aperture, which lies on the side of 

 the body immediately below the opening of the ureter. 



Tease a small portion of the ovary, or testis, in salt 

 solution or glycerine ; cover, and examine it with low and high 

 powers. 



In the female the eggs, after they escape from the genital 

 aperture, pass in large numbers into the space between 

 the two lamellse of the outer gill, which they distend very 

 greatly. Here they develop into embryos known as glocMdia, 

 which are in many respects very unhke the parent. They 

 have bivalved shells, each valve of which is triangular and 

 incurved at its apex so as to form a sharp tooth, the teeth of 

 the two valves constituting an efiBcient pair of pincers. There 

 is only a single adductor muscle ; the giUs are absent or 

 rudimentary, as also is the foot. A long coiled filament, the 

 byssus, serves to anchor the glochidium to the giU. 



Take a female in which the outer gills are distended with 

 embryos : cut the gill across ; remove some of the embryos, 

 mount them on a sUde in water, and examine them with a 

 microscope. 



III. EXAMINATION OP TRANSVEESE SECTIONS. 



Several points in the anatomy of the mussel, and notably 

 the relations of the giUs, kidneys, and heart, are best studied 

 by means of a series of transverse sections. 



For this purpose take a good-sized specimen, and put it 

 into ^ per cent, chromic acid, wedging the valves slightly open 

 so that the acid may have free access to the branchial cavity. 

 After a couple of days transfer it to spirit and keep it till 

 wanted. 



To make the sections, remove both valves of the shell care- 

 fully ; place the animal on a boa/rd, and cut it transversely 



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