250 THE PROTOCHORDATA. 



nothing in the structure or development of this part of the 

 alimentary tract in Balanoglossus which will bear compari- 

 son with the endostyle.* As indicated in the larvae of 

 Amphioxus and the Ascidians, it would seem that the 

 endostyle first became evolved or differentiated at the 

 anterior end of the pharynx, in front of the gill-slits, in 

 correlation with the dorsal position of the mouth. 



Development ; the Tornaria Larva. 



The development of Balanoglossus Kowalevskii as made 

 known to us by the admirable work of BATESON is what 

 is known as a strictly direct development ; that is to say, the 

 embryonic, larval, and adult stages follow one another by 

 gradual transitions concomitantly with the simple progres- 

 sive growth of the individual and without any striking 

 metamorphosis. In other species of Balanoglossus the 

 larval form is remarkably different from the adult, and 

 becomes transformed into the latter by a very distinct 

 metamorphosis. The extraordinary larval form here re- 

 ferred to was discovered in 1848 by JOHANNES MULLER, 

 who named it Tornaria, and regarded it, as did his succes- 

 sors KROHN, ALEXANDER AGASSIZ, and FRITZ MULLER, as 

 the larva of an Echinoderm (Starfish). 



It was not until 1869 that its true character as the larva 



* A ciliated tract in the floor of the oesophagus of a Tornaria from the 

 Pacific has recently been compared to the endostyle by W. E. RITTER. (On 

 a New Balanoglossus Larva from the Coast of California and its Possession 

 of an Endostyle. Zool. Anz. XVII. 1894. pp. 24-30.) 



The comparison is at present somewhat doubtful. More recently GARSTANG 

 has suggested that the endostyle is derived from the adoral ciliated band of the 

 Echinoderm larva. (See Fig. 119.) The suggestion is an interesting one, but 

 Garstang's idea of the relations of the pneoral lobe is very different to the one 

 here set forth. (WALTER GARSTANG, Preliminary Note on a Neiv Theory of 

 the Phylogeny of the Chordata. Zool. Anz. XVII. pp. 122-125.) 



