HISTOLOGY AND DEVELOPMENT OP MYRIOTHELA PHRYGIA. 519 



In the Triclads and Polyclads, on the other hand, the same 

 problem has been solved in a rather diflFerent fashion. The 

 development of spaces round the gut is not the most obvious 

 factj but the gut itself ramifies into a network of tubes which 

 penetrate to every part of the body. 



The Structure of the Endoderm. — Leaving these 

 general considerations, we will pass at once to a consideration 

 of the structure of the endoderm in Myriothela. 



The mouth is bounded by a thin membranous lip, or hypo- 

 stome, which is exceedingly muscular and sensitive, and is 

 probably of considerable use in seizing the prey. It is figured 

 by Allmann and Hincks in a condition of extension. In 

 fig. 20 it is shown as it appears when the animal is retracted. 



In the lip the supporting lamella is either reduced to the 

 thinnest film or is entirely absent. It is always absent from 

 about half the breadth of the lip from the free edge. The 

 ectoderm of the external surface is rich in nerve-cells, and 

 overlies a well-developed layer of radial muscles. At the free 

 edge there is no loss of histological continuity, the ectoderm 

 being continued on to its under surface for a certain distance, 

 and having the same structure. Passing downwards towards 

 the attachment of the lip, there appears at the base of this 

 ectodermal epithelium a more and more defined layer of 

 highly vacuolate cells, which are the first commencement of the 

 endoderm. In other words, the ectoderm for a short distance 

 overgrows the endoderm, thus forming a distinct zone of mixed 

 character. 



At the point where the lip merges into the tentacle-bearing 

 region the ectoderm, as a distinct structure, has finally dis- 

 appeared, and we have the arrangement shown in fig. 14. 

 Three kinds of cells may be distinguished: (1) a super- 

 ficial layer of elongated cells, staining well and uniformly 

 with picro-carmine. Each possesses a nucleus and nucleolus 

 in its basal portion, which is taperiug and wedged in between 

 the subjacent cells. In favorable preparations these cells 

 appear covered with short, fine cilia. These are, however, not 

 always visible, mainly because the animal in dying usually 



