BYSSIFEROUS MOLLUSCA. 145 



their prolongation beyond the shell, the byssus is fabri- 

 cated. 



These sea-spinners, as Reaumur calls . 25 



them,* begin their work in their earliest 

 infancy; but at that period, and for some 

 time after, if detached, they prove them- 

 selves good walkers, moving along, after 

 the fashion of other Bivalves, by the aid 

 of their extensile spinneret, to which the 

 name, but not the offices, of a foot has 

 been denied. I have already told you, 

 that some of them can voluntarily un- 

 moor, and go in quest of happier loca- 

 lities ; and if forcibly torn away from 

 their hold, they can refix the shell by the formation of 

 a new byssus, not by reattaching their old one, as seems 

 at one time to have been believed, f I believe that the 

 typical Modiolaa and the Pinnae can at no period detach 

 their byssus at will. 



There is a genus of Bivalves, from the shape of the shell 

 having a certain resemblance to a boat, named Area, which 

 contains some byssiferous species (Byssoarcae, Sw.), and 

 others that are affixed in the crevices of rocks by the foot 

 itself, changed into a sort of peduncle, dilated and corneous 

 at the extremity, to fit it for its novel function. These lead 

 us, by a gradual transition, to other Bivalves, whose ordi- 

 nation it is to be rooted, like the plant, to one spot for life, 

 and to whom a foot is denied, as indeed it is unnecessary. 

 They are fixed in various ways ; for the Author of nature, 

 in accomplishing one and the same end, ever varies his 

 means and workings. The oysters and Spondyli, horrid 

 with projecting spines, adhere by cementation, that is, with- 

 out the medium of any connecting membrane or ligament ; 

 the inferior valve, in its growth, becoming fixed and mo- 

 delled to the foreign substance on which it lies. The 

 Anomiae, which in character much resemble the oysters, are 

 fixed partly in the same way, but their chief hold is effected 

 by the transverse muscle, which, in the form of a round 

 ligament, passes through a hole in the lower valve, and is 

 firmly cemented by the intervention of a calcareous or horny 

 wafer, J In a somewhat similar manner the Terebratulae 



* " I could not doubt then that the Sea has her spinners in the mussels, 

 as the Earth has in its caterpillars and spiders." 



f Hist, de 1'Acad. Roy. des. Sc. an. 1711, p. 152. &c. Me'm. de 1'Acad. 

 an. 1717, p. 238. 



t Burguiere says, that the Terebratulae can detach themselves and swim 



L 



