62 LOW EH VERTEBRATES. 



Class II. — ACRAmA. 



" The lowest of the vertebrates," Uranchiostoma, occurs nearly everywhere in the 

 temperate and toi'rid regions along the sea coasts, in shoal waters of a couple of 

 fathoms more or less. In the United States it has been found from New York to the 

 Gulf of Mexico. Its shape and size are such that it readily escapes notice, even where 

 very common. A very large specimen might be two inches in length, three six- 

 teenths of an inch in deptli, by about one eighth in width, pointed at each end, translu- 

 cent and iridescent. Resembling a worm, it is easy to overlook or mistake it. Pallas, 

 its discoverer, 1774, called it a mollusc, and put it in the genus Limax. Costa rediscov- 

 ered it in 1834, and, calling it a fish, made it the type of the genns BrancMostoma. 

 Yarrell, in 1836, made for it the genus Amphioxus, under which name it is best known. 



Fig. 53, — Branchiosioma Inuceolatum^ amphioxus, hmcelet. 



Recent investigations have brought to light so many respects in which it differs fi-om 

 the other vertebrates, that it has been placed in a separate class, the Acrania. The 

 division is also known as Leptocardii. 



Adult laneelets, as commonly called, spend most of their time buried in the sand. 

 If one were able to watch the bottom in their haunts about nightfall in still weather, 

 he would j)0ssibly see them, where least expected, s]iring from their concealment, and 

 with a peculiar wiggling motion, in which the head itself moves from side to side, 

 swim toward the surface. After they had wandered about till tired or satisfied, he 

 would see them settle down again, and by quick bendings of the body dig into the sand 

 and throw the cover over them as they retired. Unless an occasional one, resting un- 

 easily, should rush through the sand, or, in a fright, should spring out and fi-ighten all 

 the rest, it would be hard to say just where they had disapjieared. A very close in- 

 sj)ection, however, would discover little openings here and there ; and, looking closer, 

 these would be found to be the tentacle-fringed, open mouths of the little creatures, 

 which, lying on their backs, feed u])on the minute organisms, plants, and animals, 

 drawn into the month l)y a current of water that ])asses out through the gill openings 



