408 MORGAN. [Vol. V. 



ment, without interpolation of the Tornaria. Recognizing the 

 identity of the northern species of B. Kowalevski to the southern 

 species of the Chesapeake, it became evident that the one had 

 a free-swimming, Tornaria stage, and the other a direct devel- 

 opment, or else a mistake had been made in assigning the 

 northern Tornaria to B. Kowalevski. The latter, I believe, is 

 the explanation, and for these reasons. The Tornaria came 

 suddenly and in great numbers, and were of all ages. The 

 numbers gradually diminished, then disappeared. At the time 

 of occurrence there was a decided change in the nature of the 

 tow-stuff, notably in the presence of many small hydro-medusae 

 of several species, of the larvae of Polygordius, of young Cteno- 

 phors, and small Crustacea in abundance. Many or all of these 

 must have been brought in by currents from outside ; and 

 these, like the Tornaria, remained for a time, and then disap- 

 peared. Finally, — and this in itself seems conclusive, — the 

 size of the mature eggs of B. Kowalevski of Wood's Holl corre- 

 spond exactly in size to the egg of the southern species as given 

 by Bateson. These are large and contain much yolk, while the 

 youngest Tornaria which I caught in the tow-net were only 

 about two-thirds as large as the eggs of B. Kowalevski. These 

 facts, I believe, give us sufficient reason to reject the sup- 

 posed relationship of the New England Tornaria to B. Kow- 

 alevski ; so that, as yet, we do not know the locality from which 

 the Tornaria came, unless, indeed, it prove to be identical with 

 the Tornaria found by Bourne on the English coast, and which 

 he says is identical with Tornaria Krohnii of the Mediterranean. 

 The greater part of the following work was done as holder of 

 the Adam T. Bruce Fellowship, 1890-91, in the Morphological 

 Laboratory of Professor W. K. Brooks. 



Structure of Tornaria. 



The youngest Tornaria collected are shown in PI. XXIV., Figs. 

 1, 2, 3, and measured about \ mm. in length. The larva is very 

 transparent ; and the large, thin-walled digestive tract is seen 

 within the animal. This is already divided into its three com- 

 partments, as seen in the figures. The anterior of these is the 

 oesophagus, and is a flat tube connecting the mouth with the 

 next division, or so-called stomach. This is large, and fills up 



