434 
W. A. HASWf]LL. 
first form its entire thickness, with the exception of the 
exceedingly thin internal epithelium. Bnt afterwards mus- 
cular fibres ai’e developed both internally and externally. 
The mode of formation of these is not clear, but since no 
other elements come into play, tliere can be little doubt that 
a portion of the cells of the wall of the pharynx are of the 
nature of myoblasts. The rest become the nerve-elements 
and the glandular and excretory cells. 
The mode of formation of the so-called epithelium of the 
pharynx is a matter of some interest. It is represented at 
first, as already pointed out, by the thin syncytial lining of 
that part of the endoccele from which the pharynx becomes 
developed ; and at no subsequent stage does its epithelial 
character become more pronounced. On the contrary, as 
development advances, the cellular character of this layer 
becomes almost or completely lost. In fact, were it not for 
the occasional occurrence in it of a more or less altered 
nucleus, it might be supposed to be entirely non-cell ular. 
But, though it loses, or nearly loses, its cellular character, 
this layer greatly increases in thickness, and, in the adult, 
maintains its thickness in spite of the loss of material to 
which it must be subjected as a result of constant wear and 
tear during the capture and ingestion of active living prey. 
Up to a late stage in Temnocephala fas data the rest 
of the digestive system is merely represented by a short 
passage, which continues back the lumen of the pharynx, 
and ends abruptly within the mass of yolk. This is the 
future oesophagus, and in its neighbourhood are a number of 
cells destined to become the unicellular or “ salivary ” 
glands. Thus, at a stage when the muscular wall of the 
pharynx, with its anterior and posterior sphincters, has 
reached an advanced stage of development, the intestine is 
not yet definitely represented. 
At this stage the cavity of the pharynx is still completely 
shut off from the exterior by the septum formed, as already 
described, from the persistent thin roof of the original endo- 
coele. 
