348 VIVIPAROUS BIVALVES. 



scatter themselves abroad as instinct leads or chance directs, 

 to continue their race in new localities. 



A few genera amongst the bivalves are viviparous, that 

 is to say, the ova are carried from the ovary to be deposited 

 in the interstices of the external laminae of the branchiae, 

 where they are retained until the young are hatched and 

 have acquired all the perfection of maturity. The pond and 

 river mussels have this character, which is still more evi- 

 dently developed in the little fresh-water Cyclades, and in 

 the pretty sea-shore Kelliadae. In the Unio and Anodonta, 

 says Mr. Garner, the ova may be found for several months 

 in the external branchiae after leaving the ovaria, distending 

 their organs in a remarkable manner. * The Cyclades had 

 also been known to be viviparous for several years before 

 Professor S. Nilsson discovered the peculiarity of organiza- 

 tion provided for their temporary retention in Cyclas cornea. 

 This is a peculiar membranous sac fixed above to the root of 

 the branchiae, and free in the gill cavity below. There are 

 from two to five young in each little sac attached by a capil- 

 lary navel-string holding the position of the byssus in other 

 bivalves ; and although the young in each sac are all alike 

 and equal, yet young, various in age and in size, are found 

 not only within the same shell, but even in the same gill.f 

 The Kellia has its peculiar apparatus also. All other bi- 

 valves have the siphons, when there are any, at the posterior 

 end of the shell, but the Kellia has a large siphonal tube in 

 front as well as a short one behind, as was first pointed out 

 by Mr. Alder. J Now there has been a good deal of ingeni- 

 ous disputation as to the function of this front siphon, but 

 the anomalousness of its position seems at length to have 

 been explained away by Mr. W. Clark, who has made the 

 interesting discovery that it is an oviduct ; and not merely so, 

 but for some time a nidus also for the full developement of 

 the testaceous young. " But," says Mr. Clark, " I shall not 

 be surprised to find that Mr. Alder and myself have mis- 

 taken the use of this fold in Kellia rubra, and that it may 

 minister to supply water to the viviparous colony deposited 

 in the ovarium of the animal of this species, and also act 

 as an oviduct and receptacle for the young, until they are 

 sufficiently developed for exclusion. This idea arises from 



* Charlesworth's Mag. Nat. Hist. iii. 441. 



t Mollus. Suecise terr. et fluv. 97. This reminds one of the curious 

 ovarian sacs in the Unio irroratus as described and figured by Lea. On the 

 genus Unio, p. 13, pi. 5, fig. 6, 7. See also Sowerby's description of Car- 

 dita concamerata in Zool. Journal, iii. 526. 



J Trans. Tyneside Nat Club, i. 188. 



