[228] I have made no attempt to discuss the homologies of the plates of the larval Echinodermata because the criteria for such a discussion are still in dispute. The suggestive memoirs of P. H. Carpenter (No. [548]) on this subject may be consulted by the reader. Carpenter attempts to found his homologies on the relation of the plates to the primitive peritoneal vesicles, and I am inclined to believe that this method of dealing with these homologies is the right one. Ludwig (No. [559]) by regarding the opening of the madreporic canal as a fixed point has arrived at very different results.
[229] The dates in this reference are the dates of publication.
CHAPTER XXI.
ENTEROPNEUSTA.
The larva of Balanoglossus is known as Tornaria. The præ-larval development is not known, and the youngest stage ([fig. 272]) so far described (Götte, No. [569]) has many remarkable points of resemblance to a young Bipinnaria.
Fig. 272. Early stage in the development of Tornaria. (After Götte.)
W. so-called water-vascular vesicle developing as an outgrowth of the mesenteron; m. mouth; an. anus.