290 LECTURE XXI. 



they quit the marsupium, and may be observed swimming freely about 

 in the cavity of the external gill. They were mistaken for parasitical 

 animals by Rathke, and described by him under the name Glochidium; 

 and the difference of form between the young and the old Anodontes 

 was such, that Professor Jacobson adopted the same opinion, con- 

 sidering it to be improbable that they could be the offspring of the 

 animal in which they were found. In fact, besides the byssiform ap- 

 pendage which characterises the young Anodon^ the valves are, at first, 

 triangular, with one side short and straight, where they are united by 

 the ligament, and the other two sides meet, and terminate in a point, 

 beyond which a process of the mantle is continued, which is dentated . 

 on its outer surface : two pointed processes project from the inner 

 surfaces of the valves. The entire amount of change effected in the 

 progressive acquisition of the mature form merits the name of a me- 

 tamorphosis. 



The course of the intrabranchial development of the young Anodon 

 extends over two, three, or four months : the young sometimes escape 

 from the parent as late as in September. The young animal, after 

 exclusion^ uses the prehensile or adhesive filament to anchor itself to 

 the shell of the parent, or to some foreign body. 



In the genus Cyclas, ]Mr. Garner * has observed from ten to twenty 

 of the young fry, situated in the internal branchiae : they are discharged 

 one by one when they attain about the sixth of an inch in diameter. 

 The oviducts in the Cyclas open over these internal branchiae, and 

 are only accessible to the w^ater from behind, as are the external 

 branchiae of the Unio. The young Cydades are sometimes found 

 adhering by a byssus to different parts of the body of the parent. 

 The young of the genus Naias have been observed to anchor them- 

 selves after exclusion from the parent, by a byssus, which is usually 

 wanting in the large and full-grown animals. 



This temporary means of attachment must prevent many of the 

 young and feeble bivalves from being carried away by the stream at 

 a period when their shell has not attained sufficient hardness to 

 protect them from the numerous predatory aquatic animals to whose 

 attacks they would be exposed ; and we may thus discern, in the de- 

 ciduous byssus, an evidence of prospective design for the well-being 

 of the weak and defenceless. 



* Zoological Transactions, vol. ii. p. 97, 



