1901.] N^\TURAL SCIKJS'CES OF PHILADELPHIA. 693 



behind the usual point of termination. The same may be said of 

 Zygeupolla. The exjianded rhynchoca-l is constricted in the region 

 of the inner circular umsclc at the beginning of the middle intes- 

 tine, and instead of ending here, as may have been the case primi- 

 tively, has secondarily developed backward through the whole 

 length of the body. 



6. The Proboscis. — The proboscis, Plate XL, tigs. 1, 16, P., 

 is attached to the body vrall in the brain region, just anterior to 

 the dorsal commissure. It lies in the rhynchoctel, Re, bathed by 

 the rhynchocoelomic fluid. The posterior end is not attached to the 

 wall of the rhynchoc«d but is entirely free, there being no retractor 

 muscle. 



In regard to its histology the proboscis may be divided into 

 three regions, which, however, pass very gradually into one an- 

 other. These parts are (1 ) the anterior region, which is compara- 

 tively short, being of about the same length as the (lesophagus, 

 above which it lies; (2) the middle region, comprising the greater 

 part of the proboscis; and (3) the extreme posterior region, only 

 a few millimeters in length. 



In the following description the nomenclature of Burger (1895) 

 will be followed, by which in the unevaginated proboscis the layers 

 nearest the central cavity are termed the inner layers, those toward 

 the periphery the outer. 



The Anterior lieyiou. — This part of the proboscis is usually 

 straight and the average diameter is 0.116 mm. A cross section, 

 Plate XLII, fig. 85, has the following layers: 



1. The outer epithelium, o.Ej). 



2. The subepithehal layer of circular muscle fibrils, Ep.M.J. 



3. The outer basement layer, o.B.L. 



4. The longitudinal muscle layer, L.M. 



5. The lateral nerves, P.N. 



6. The inner basement layer, l.B.L. 



7. The inner epithelium, i.Ep. 



The outer epithelium, fig. 44, o.Ep., in the most anterior part 

 of the anterior region, consists of low, rather brick-shaped cells, 

 in which the cytoplasm is quite abundant, and whose nuclei are 

 large and prominent. Farther back the cells are lower and 

 finally form a flattened endothelium, that is frequently toi-n away 

 in my preparations. 



