182 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



need not be described. When this change has taken place the 

 shell is formed as a continued saddle-shaped plate on the dorsal 

 surface, where subsequently the two valves become separated. 

 They are at first rounded but become triangular, and an avicular 

 or bird's-beak organ is formed at the top of each valve. It has 

 serrations at its edge which aid in affixing it as a parasite. 

 More than this from actual observation cannot be said. In the 

 European Unio, after the shell is formed, a new structure makes 

 its appearance which is known as the byssus gland. The following 

 is what succeeds, according to Rabl as given by Balfour* : — Before 

 the mantle are fully-formed peculiar sense-organs, usually four in 

 number, making their appearance on each lobe. Each of them 

 consists of a columnar cell, bearing at its free end a cuticle from 

 which numerous fine bristles proceed. Covering the cell and the 

 parts adjoining is a delicate membrane, perforated for the passage 

 of the bristles. The largest and first formed of these organs is 

 placed near the anterior and dorsal part of the mantle. These 

 organs probably have the function of enabling the larva to detect 

 the passage of a fish in its vicinity, and to assist it therefore in 

 attaching itself. With the development of the shell, the mantle, 

 and the sense-organs, the young mussel reaches it full larval 

 development, and is now known as a Glochidium. 



If the parent with Glochidia in its gills, is placed in a tank 

 with fish, it very soon (as I have found from numerous 

 experiments) ejects the larvae from its gills, and as soon as this 

 occurs the larvae become free from the egg-membrane, attach 

 themselves by the byssus cord, and when suspended in this 

 position continually close and open their shells by the contraction 

 of the adductor muscle. If the mussels are not placed in a tank 

 with fish the larvae may remain for a long time in the gills. 



Acmcea septiformis Q. & G. — In this case the ova form no 

 exception to what I have observed in Ostrea and what has been 

 generally seen amongst the Mollusca. In dissecting away some 

 of the gill-plumes I have found the eggs associated with tho 

 branchial apparatus. They are round lenticular vesicules of 

 somewhat dark leaden grey colour containing the partly developed 

 embryo, in which I have never been able to make out clearly 

 more than a highly crystalline oval mass, filling about one-third 

 of the membrane, which is possibly the differentiation of the shell. 

 The eggs thickly surround the gill-plume, but are not, except 

 in a few instances, within the margin of the plates. In the 

 plume there is, properly speaking, no gill-chamber, as in the 

 Lamellibranchiata. The filaments are long and narrow, and the 



C. Eabl, " Ueber d. Entwicklungsgeschichte d. Malermusohel," Jenaische 

 Zeitschrift, Yol. X., 1876. 



