No. 3.] GROWTH AND METAMORPHOSIS OF TORN ARIA. 431 



backwards in the middle of the Tornaria to the level of the cir- 

 cular band, where it is separated from the intestine by the dia- 

 phragm-like partition. The intestine runs for a short distance, 

 and ends at the anus. 



Sections show that the anterior enteroccel is well developed, 

 and corresponds to the stage shown in Fig. 6. There is no 

 trace as yet of the second and third body cavities, but the whole 

 blastocoel cavity between digestive tract and ectoderm is filled 

 with a gelatinous substance. Weldon figures the body cavities 

 in older stages, claiming them to be exactly similar to those of 

 the New England Tornaria. I infer that these body cavities have 

 not yet developed from the endoderm in the larvae I studied. 

 A description of the other points in the anatomy and histology 

 of the Nassau larva would be largely a repetition of the pre- 

 ceding pages. The collection of mesenchyme cells on the outer 

 wall of the oesophagus is very marked. They seem to form an 

 anastomosing network over its whole surface. 



Historical and Critical. 



The free-swimming larva was first described and figured by 

 Johann M tiller, in the year 1848, and was believed to be the larva 

 of an Echinoderm. He gave to it its present name because 

 "das Thierchen . . . dreht sich immer, fort im Kreise und 

 durch die Thatigkeit seinem Wimpern. Daher es den Namen 

 Tornaria fuhren mag." 



Mtiiler saw and described all of the principal organs of 

 the larva, including digestive tract, anterior enteroccel and its 

 madreporic-like opening, the posterior body cavities, ciliated 

 bands, apical plate and its muscle band. It seems to me prob- 

 able that his somewhat diagrammatic figures of the apical plate 

 may not be quite exact. The longitudinal ciliated band is 

 described as crossing over from right to left at the apical plate 

 in very young forms, thus separating off, as in Bipennaria, an 

 anterior isolated "plastron." 



In the preceding sections I have given my reasons for 

 believing that at first the longitudinal band is a continuous 

 circumoral band, and secondarily breaks, with a tendency to 

 form two bands. Mtiller's extensive knowledge of Echinoderm 

 larvae helped him greatly to understand the structure of Tor- 



