1914] Sharp: Diplodinium ecaudatum 67 



to the last one-fourth of the body, where they, too, disappear. It is 

 to be noted that all traces of oesophageal structure are lost at about 

 the same level as that at which the united ventral and right skeletal 

 structures disappear. 



That the above described structure functions as a true skeletal 

 (supporting) structure, not only for the retractile oseophagus but 

 also for the entire body, seems altogether certain. In the consideration 

 of the oesophagus and the motile anterior end of the body additional 

 evidence will be given for this conclusion. 



Giinther (1899, p. 553) describes a homologous structure for 

 Ophryoscolex caudatus, which is composed of two parts, at first sep- 

 arated one from the other, partly embracing the gullet and then 

 approaching each other quite closely, finally to separate again and 

 end singly, deep down in the lower part of the animal. In the case 

 of Ophryoscolex caudatus this structure is described as being com- 

 posed of two layers, a fine, thin, structureless membrane and an en- 

 closed substance which shows a definite structure, "mit grofien, meist 

 zur Langsachse des Stiitzsapparates senkrecht gerichteten Waben." 

 And in his description of a similar structure in Entodinium rostratum 

 Giinther (1900, p. 644) says: "Dicht unter der Pellicula im oberen 

 Theil des Thiers beginnend, rechts vom Kern liegend, zieht sich das- 

 selbe (Fig. 13 st) in ziemlich breiter Ausdehnung bis zur Mitte, wo es 

 sich in 3 Theile theilt (Fig. 14 st), von denen jeder einzeln tief unten 

 im Thier endet, stets der Pellicula anliegend. " Of the finer structure 

 of the apparatus in this case nothing is said. 



It is hardly necessary to point out that this description of the 

 "Stiitzapparat" in Ophryoscolex caudatus has little in common with 

 the above description of the skeletal structure in Diplodinium ecau- 

 datum except in general indications of homology. 



Boundary layer. Separating the ectoplasm from the entoplasm is 

 a constant and well-defined boundary layer (bd. I., pi. 4, figs. 3, 5, 

 pi. 6, fig. 15, and pi. 7, fig. 25). This layer is probably ectoplasmic 

 in nature; it stains very heavily either with iron haemotoxylin or 

 Mallory's connective tissue stain. Even in well-stained whole mounts 

 it may be clearly made out. In fact it may easily be mistaken for an 

 external structure, it shows so clearly. From the alveolar ectoplasm 

 this boundary layer is separated by a thin layer of regularly placed, 

 small, cubical alveoli, and from the more homogeneous entoplasm by 

 a similar alveolar layer consisting of large, more definitely cubical 

 alveqji. This boundary layer with its two investing layers of alveoli 



