RECENT WOEK ON THE PEOTOCHORDA. 239 



A word is necessary as to the development of the Entero- 

 pneusta and the significance of the direct and indirect methods 

 of development. In the text-book before us the authors 

 consider the direct development as the more typical. This may 

 be so in a certain sense, but it is necessary to bear certain facts 

 in mind. 



Spengel showed clearly that the Enteropneusta are divisible 

 into three families, but he only named one of them, namely, the 

 Ptychoderidse. I propose to call the other two families the Spen- 

 gelidse and Balanoglossidse respectively. Spengel was of the 

 opinion that the Balanoglossidas comprise the most primitive 

 forms, and the Ptychoderidse the highest or least primitive 

 forms. As I have before stated, I hold the Ptychoderidae to be 

 the primitive family, emphatically not the Balanoglossidae ; I 

 think the anatomy ofPtychodera flava shows this conclu- 

 sively. 



The Balanoglossidse are all northern forms (White Sea, 

 Greenland, Canada, Massachusetts), with relatively large eggs 

 (from -^ to 1|- mm. in diameter), and from the size of the egg 

 alone we are justified in concluding that they develop directly 

 in the manner described by Bateson in B. kowalevskii. The 

 Ptychoderidse^ have small eggs (rarely more than ^ mm. in 

 diameter) which develop into a Tornaria larva, i. e. in- 

 directly. 



have retained some of the characters of a primitive creature, just as the 

 Ascidian tadpole retains primitive features which liave quite disappeared from 

 the larva of Amphioxus. 



The difficulty will naturally arise as to how the portion of the primitive 

 gut, represented in the stomochord, could have been projected past the 

 mouth. I do not think we are obliged to make an obstacle of this difficulty. 

 The principle of segregation will temporarily account for it. That segregation 

 has taken place is shown by the origin of such a complex structure as the 

 stomochord from a simple primordium (the mode of development of the vermi- 

 form process of the stomochord in Spengelidee is unknown). 



There is so much on the surface which demands explanation that I have 

 ventured on dangerous ground in the endeavour to collate the various facts. 



' Probably also the Spengelidse (Schizocardium, Spengelia, Glandi- 

 ceps). 



VOL. 42, PART 2. NEW SERIES. Q 



