LATER LARVAL DEVELOPMENT OP AMPHIOXUS. 185 



It will be remembered as a curious circumstance that the adult 

 habitually assumes a similar vertical position in the sand.^ 



The time of life at which the larva seeks its home in the sand 

 varies greatly. While, on the one hand, I have dredged all the 

 stages described below pelagically, I have, on the other hand, 

 found larvae which had only reached the third stage of the later 

 larval development (see below ; also PI. XIII, fig. 4) in the sand. 



Resume of the Entire Development of Amphioxus. 



The various phases in the development of Amphioxus may 

 be conveniently arranged in the following way : 



I. The Period of Embryonic Development comprising the 

 first thirty-two hours. It commences with the segmentation 

 of the ovum, and ends with the formation of the mouth and 

 first gill -cleft. According to the habit of the embryo, this 

 period may be subdivided into — 



(a) The time — namely, the first eight hours — during which 

 the rapidly developing embryo is confined within the vitelline 

 membrane, the successive stages being marked by the progress 

 of the segmentation and gastrulation, the commencement of 

 the formation of the myocoeloraic pouches, the diff'erentiation of 

 the medullary plate, and the formation of the neurenteric canal. 



(6) The time which elapses between the emergence of the 

 ciliated embryo from the vitelline membrane, and the appearance 

 of the mouth, first gill-cleft, and anus — the stages being marked 

 by the successive formation of myocoelomic or archenteric 

 pouches to the number of fourteen pairs. The myotomes 



' The accounts of Amphioxus giveu by Rathke (Konigsberg, 1841) and J. 

 Miiller (Berlin, 1844) have given rise to the impression that the usual status 

 quo of Amphioxus is, to be lying on its side on the sand. As stated above, 

 this is not the case. If it were so, it might be supposed to offer a simple 

 explanation of the one-sided character of the larva — not that I think it would, 

 however. J. Miiller, indeed, says that Amphioxus is fond of burying the caudal 

 half of its trunk in the sand, as an occasional divertissement. The lying 

 on one side, however, is what is occasionally done; and that is the result, 

 not so much of a subtle inherited tendency to assume that position of rest, as 

 of a gross incapacity on the part of Amphioxus to maintain its equilibrium in 

 any other way when out of the sand. 



