432 MORGAN. [Vol. V. 



naria, but at the same time may have led him to overlook some 

 of the differences in the two. 



Krohn, in 1853, described as a Holothurian larva a Tornaria, 

 at the time of metamorphosis. In 1866, Alexander Agassiz 

 described the New England Tornaria — the same considered in 

 the preceding pages. He thought Tornaria was the larva of 

 some Echinoderm, probably of a very primitive starfish, as it 

 retains permanently the embryonic features of Brachiolaria. At 

 the same time he pointed out that Tornaria differs in some 

 essential points from all Echinoderm larvae ; viz. in the circular 

 band of cilia, and peculiarities of the water system. He did not 

 state, as Metschnikoff later said he did, that he had seen the 

 anterior enteroccel arise from the digestive tract, but says 

 explicitly that " the mode of formation of the water system, 

 though I have not traced it, is probably similar to that of the 

 Brachiolaria as a diverticulum of the digestive tract." 



Fritz M tiller 1 referred the larva to the Echinoderms, and dis- 

 covered the proboscis vesicle, which he believed to be the 

 earliest representative of the heart. E. Metschnikoff described 

 in 1866 a small worm-like larva, which he caught in the tow net 

 at Naples. He points out that it is the young of Balanoglossus, 

 but did not recognize its connection with Tornaria. The 

 anterior enteroccel he misinterpreted as possibly the nervous 

 system, and affirms that gill slits are absent. Nevertheless, in 

 the single figure accompanying the paper he distinctly figures 

 a pair of gill pouches, but wrongly believed them to represent 

 liver sacs. The peculiarities of Balanoglossus he thought were 

 so great that one had best consider the animal the representative 

 of a small group most nearly related to the Annelids, and 

 pointed out that Keferstein's homology of the proboscis to the 

 Nemertian proboscis is false. Metschnikoff thought that the 

 proboscis of Balanoglossus is homologous to the cephalic lobes 

 of Annelids. The larva which he caught is to be compared to 

 an Annelid larva of the Mesotrochal type. Kowalevsky's paper 

 on the anatomy of the adult appeared in 1866. This gave the 

 first exact account of the adult structures, overlooking, however, 

 the dorsal nerve chord. Delle Chaije had discovered Balano- 

 glossus in 1826, and Keferstein had given a description of it in 



1 I have not seen this paper, and quote on the authority of Metschnikoff and 

 Spengel. 



