A. E. Verrill — Study of the family Pectinidce. 47 



may have retained, more completely than at present, a free-swim- 

 ming or pelagic life. There may, very likely, have been small forms 

 that retained the velum through life and used the valvular method 

 of swimming when a more rapid motion than the action of cilia 

 could give was required, in order to avoid enemies. 



According to Dr. Jackson's observations on the young of C. irra- 

 dians, the spat, at first, creeps about with its foot before it is able to 

 swnm by the valvular method, but even in that stage the byssal 

 groove is present.' (PI. xx, figs. 1, 2.) 



In any case, it is probable that the first form of bivalve foot to be 

 developed, in the later veliger-stages of primitive bivalves, would 

 have been a simple foot adapted to adhesion to floating objects or to 

 stationary algoe, etc., and not a foot adapted to creeping over the 

 muddy bottom as Mr. Jackson has assumed, when considering the 

 Nucula-like forms as the most primitive of bivalves. 



We may suppose that the earliest form of adhesion was temporary, 

 and merely for the purpose of rest during the veliger condition, and 

 it may have been effected by means of the mere surface adhesion of 

 a little specialized, soft, fleshy or tongue-shaped foot, aided, perhaps, 

 by a secretion of mucus from the surface. Such a mode of adhesion 

 to objects is common among planarians, small nemerteans, annelids, 

 and the young forms of many groups, at the present time. 



From such a primitive adhesive foot the transition to a larger foot 

 with more specialized cells situated in a groove for the secretion of 

 stronger byssus-like threads of mucus would have been easy. 



Such threads of adhesive mucus are formed by the foot glands of 

 many laud slugs and by certain marine species at the present time 

 (e. g. Litiopa bombyx, a small gastropod that attaches itself to float- 

 ing sargassum in this way). 



From this structure of foot the transition would have been easy 



^ The form of this foot is like that of Mytilidte, in which the foot is used for climb- 

 •.ng about and forming a byssus. 



Dr. .Tackson states that his youngest spat were not attached by a byssus, but crept 

 about by means of the long, ligulate, grooved foot, and seemed incapable of swim- 

 ming. He also observed that the spat could use the marginal tentacles for creeping 

 about and clinging to objects. Those spat that I have observed (apparently quite 

 as young) were capable of attaching themselves by a byssus, and when slightly older 

 were seen to swim. When kept in still water in vessels, such spat may be much 

 more mactive than when living in open waters, in coustant motion. The form of the 

 foot of the young spat, as described by Dr. Jackson, is better adapted for climbing 

 over and adhering to sea-weeds than to creeping on the bottom, and requires far less 

 specialization than does a disk-hkc or flattened muscular foot for creeping purposes. 



