LAMELLTBKANCmATA. 527 



young Anodon, when first excluded, is armed, may serve to defend it 

 against some small persecutor or other. 



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

 of the young fry, situated in the internal branchiae, where each has 

 its special compartment : they are discharged one by one when they 

 attain about the sixth of an incb in diameter. The oviducts in the 

 Cyclas open over these internal branchiae, and are only accessible to 

 the water from behind, as are the external branchiae of the U7iio. 

 The young Cy eludes 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 themselves, 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 pro- 

 tect them from the numerous predatory aquatic animals to w^iose 

 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. 



Prof. Edward Forbes, in an interesting paper published in tlie 

 " Edinburgh New Philosophical Journal," has cited some facts of the 

 periodical cessation of development of bivalves in certain shell-banks, 

 and has alluded to the knowledge of such phenomena as being familiar 

 to fishermen, who express it as being a " shifting "of shell-beds, which 

 they erroneously attribute to the moving away and swimming off of 

 a whole body of shell-fish, such as mussels and oysters. But even 

 the Pectens, though much better endowed with locomotive power 

 than the cemented oysters or byssus- bound mussels, have very little 

 power of progressing to any distance when fully developed. With 

 respect, however, to Prof. Forbes' opinion, that "the shiftino- or 

 migration is accomplished by the young animals when in a larva 

 state," this cannot be accepted as one of general application. The 

 facts, at least, that have been ascertained in regard to the develop- 

 ment of fresh-water bivalves are directly opposed to such migratory 

 powders in the "larvse;" for these are distinguished by organs of at- 

 tachment which are peculiar to the immature state of the bivalve, 

 and fetter them to their birthplace until, in the progress of o-rowth, 

 they attain freedom with maturity. I confess that I entertain great 

 doubts whether the so-called " shifting " of shell-beds be the result 

 of a migration en masse of the mussels or oysters, either in their 

 larval or mature states ; but deem it more probable that the disap- 



* CCCXIV. p. 97. 



