78 University of California Publications in Zoology [Vol. 13 



appears, and so leaves the left or internal side of the oesophagus in 

 free communication with the entoplasm {oes., pi. 4, figs. 3-5; pi. 7, 

 figs. 28-30). The walls of the oesophagus show, according to Eberlein 

 (1895, pp. 245 and 255) three layers ("Gewebsschichten") : (1) an 

 inner layer which is turned toward the lumen and which is a thin 

 continuation of the cuticle; (2) the middle layer, which is formed 

 by the ectoplasm and is characterized by a closer and more regular 

 formation of the reticulum; and (3) an outer layer which is formed 

 by the boundary layer previously described. The definite arrangement 

 of the "Gewebesschichten" which Eberlein (1895) describes for the 

 "Schlund" of Ophryoscolex inermis and refers to as being the same 

 for Diplodinium maggii (p. 255), I am sorry to say, I cannot confirm 

 for D. ecaudatum. The oesophagus, as is shown in longitudinal sec- 

 tions, does not come into contact with this third or boundary layer 

 until it has descended some distance through the ectoplasm (oes., pi. 4, 

 fig. 3; pi. 6, fig. 15). According to my observations, the walls of the 

 oesophagus are composed (1) of a thin cuticular continuation from the 

 cuticle of the body (pi. 4, fig. 3); (2) of definite longi- 

 tudinal strands (oes. retr. str., fig. B; pi. 4, figs. 3-5; pi. 7, 

 figs. 29, 33), w^hich, since they are attached posteriorly to the fused 

 ventral and right skeletal structures, would seem, both from their 

 structural connections and their contractile nature, to function essen- 

 tially as retractor strands; (3) of certain oesophageal fibers (oes. f., 

 fig. B; pi. 4, fig. 4, and pi. 7, figs. 23-25), which are deemed neural 

 in function and which will be described later; and (4) of the ground 

 substance or matrix of the oesophagus, ectoplasmic in nature and to 

 w^hich or in which the above structures are attached. 



The number of these oesophageal retractor strands is so large, 

 their extent so great, their arrangement so complicated, and their func- 

 tion so important that it seems best to give them a further and more 

 detailed consideration. Several estimations indicate that there are 

 from 100 to 150 of these retractor strands in the oesophageal walls. 

 They appear in cross sections (oes. retr. str., pi. 4, fig. 5) as delicate 

 radial lines joining the inner and outer lamellae of the oesophageal 

 wall and in longitudinal sections as little ribbon-like bands which 

 extend from the oral opening to the extreme posterior limit of the ento- 

 plasm. In fact it seems probable that a number of these strands end 

 in the vicinity of the anus. A satisfactory analysis of their arrange- 

 ment has been made possible through a comparative study of the 

 oesophageal retractor strands in Diplodinium bursa, which is a some- 



