442 ANATOMY OF THE LAMELLIBRANCHIATA. 



of Glochidium. He is followed in this opinion by Jabobson, ■ 

 who considers their appearance and structure to preclude the 

 possibility of their being the young of the animal in which 

 they are found. The valves are different in shape, being tri- 

 angular, with the ligament at the short straight side, the other 

 two sides terminating in a point, at which, in each valve, we 

 see a process of membrane, dentated on its exterior surface. 

 Two pointed processes also appear projecting from the inner 

 surface of the valves. There is no foot developed, and the 

 muscles are situated on the dorsal edge of the valves, and al- 

 low them to be opened to a great extent. On inspecting a 

 very young Unio we see the valves commenced by triangular 

 nuclei, of the same shape as described above in the embryo ; 

 and it is by the greater development of the posterior extremi- 

 ty of the valves, that they gain the form of the full-grown 

 muscle. The membranes at the points of the valves may be 

 incipient branchiae and tentacles, the other processes are pro- 

 bably the nuclei of the teeth of the valve. Home does not 

 seem to have known the true oviducts, as he considers the 

 interlaminar spaces of the branchiae to be such. Bojanus 

 considers the branchiae as uteri, or receptacles for the ova. — 

 Joerg 1 calls the internal branchiae ovaria, and the external 

 ones testes. The Anodonta anatina and An. cygnea are both 

 viviparous, as well as the Uniones, though Draparnaud, 3 on 

 the authority of Poiret, denies that the former is so. In the 

 different species of Cyclas we find from ten to twenty of the 

 young, some very minute, others much larger, situated in the 

 internal branchiae on each side. 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 bran- 

 chiae, and they are only accessible to the water from behind, 

 as are the external ones of the Unio. Three or four of these 

 young animals are inclosed in a membranous case, but the 

 largest are found separate. They sometimes also adhere by 

 a byssus to different parts of the body of the parent. Turton 4 

 says that in the month of June he has found the old animals 

 of the Kellia rubra containing about twelve perfectly formed 

 young ones ; the author knows of no other instances than 

 those mentioned, of viviparous bivalves. Some species seem 

 to employ the foot for fixing the ova. 



In no case are the ova discharged by the mouth, i or by the 

 anus. 6 Nor is there, in those which are viviparous, any duct 



'See paper by Blainville, Ann. Sci. Nat. vol. xiv. 



2 Meckel, Comp. Anat. par Jourdain, vol. i. 3 Hist, ties Molliisques. 



4 Fleming, Brit. Animals. 5 Treviranns, Zeitschrift. vol. i. 



6 Cams, Lehrbuoh, vol. ii. 



