1914] Sharp: Diplodinium ecaudatum 91 



schiedenheiten dadurch vor, dass es mehr vom Riicken gesehen darge- 

 stellt wurde. ' ' Hence the Diplodinium rostratum Fiorentini described 

 by Eberlein also falls into the synonymy of Diplodinium ecaudatum 

 forma caudatum. 



The action of Eberlein (1895) in using the name Diplodinium 

 caudatum for a new species discovered by him has already been dis- 

 cussed. 



After an exhaustive study of the living animals, whole mounts, and 

 transverse, frontal, sagittal, and oblique sections of all the forms of 

 Diplodinium ecaudatum, both from the originals, camera drawings, and 

 microphotographs, it is certain that with the exceptions of normal 

 variations and of the changes in form and structure of the posterior 

 extremity occasioned by the presence of one or more spines, the mor- 

 phology of all of these forms is identical. Hence it is only necessary 

 in this place to consider these spines and the changes in form and 

 structure which their presence occasions. 



Forma caudatum (pi. 5, fig. 6) appears in almost as great num- 

 bers as does forma ecaudatum. This form may be immediately distin- 

 guished from all the other forms of this species by the fact that a 

 portion of the posterior end of the body is prolonged in the form of a 

 tail-like continuation, or spine {sp. 1, pi. 5, fig. 6). This spine, which 

 is designated as the primary spine, takes its origin from the whole of 

 that portion of the posterior end of the body which lies ventral to the 

 anal opening. Cross-sections of the spine at or near its base appear 

 bean- or kidney-shaped, with the convex margin directed ventrally 

 and the concave side directed dorsally; as the distal extremity is ap- 

 proached, the cross-sections become more nearly circular. Eberlein 

 (1895, p. 262) describes this spine as being ''von beiden Seiten etwas 

 zusammengedriicktes, " a description which is not substantiated by 

 cross-sections of the spine. In life this spine is equal to from one- 

 third to one-half of the length of the body and is either straight or 

 slightly curved distally toward the dorsal side. In fixed specimens 

 it is universally more or less curved toward the dorsal side. The 

 cuticle, somewhat thickened, completely covers the spine and the 

 ectoplasm is prolonged down into it. In no other way does the presence 

 of this spine affect the morphology of the animal. 



In swimming this spine seems to function as a rudder, for it is to 

 be noticed that D. e. forma caudatum is able to advance in a more 

 nearly straight line than is D. e. forma ecaudatum. Dimensions of this 

 form are given below, page 95. 



