2 MORPHOLOGY OF 



vented by illness from making any additional observations on 

 its development. 



On leaving Beaufort^ N. C, I returned to Hampton, Virginia, 

 in the beginning of September, in the hope of finding the later 

 stages in the development of B. Kowalevskii, and was 

 fortunate enough to procure a complete series of larvae from 

 Stage H to individuals possessing ten gill-slits, in which con- 

 dition the generative organs are first present. It is intended in 

 this paper to give a general account of these stages, together 

 with the histology of the animal, until two pairs of gill-slits are 

 developed (fig. 1). From this point the further histological 

 differentiation of the various organs will be described under 

 separate headings. 



With this account of the organogeny of B. Kowalevskii 

 will be given, as far as possible, a comparative description of the 

 same parts in all the species which I have hitherto examined. 



External Changes. — The larva of B. Kowalevskii was 

 described (loc. cit.) as having the egg in an elliptical form, 

 divided by two transverse constrictions into three segments. 

 The surface of the body was ciliated, a special tuft of long 

 cilia being developed at the anterior end, while the posterior 

 region was surrounded by a transverse band of long cilia. 



From further observations it seems probable that this period 

 (Stage D) assigned as the time of hatching is too early ; for 

 embryos kept in aquaria do not break the membranous shell 

 before Stage G is reached. Probably, therefore, the larvae 

 found swimming in Stage D had escaped owing to an artificial 

 rupture of the shell during the process by which they were 

 found, an account of which is given in an appendix. 



The formation of the mouth as a ventral pore in the an- 

 terior groove was described (No. 3, fig. 41). It opens directly 

 into the archenteron, which was previously a closed sac, from 

 which five mesoblastic pouches had been given ofij forming 

 the anterior, middle, and posterior body cavities respectively. 



The external relations of the parts then become changed 

 until finally the larva has the shape shown in No. 3, figs. 16 

 and 17. In this stage the proboscis is conical and the middle 



