STUDIES ON THE TURBELLARIA. 433 
In Heterocherus, as will be rendered clear by fig. 8, 
representing a section passing through the mouth, the latter 
is an opening leading directly inwards—the integument and 
muscular layers being involuted round its margin to a very 
slight extent to form a very short passage, but not becoming 
modified in any way, except that the integumentary glands 
are shghtly more numerous. 
There is no intestinal epithelium—the food being received 
into a mass of nucleated protoplasmic material, which is 
devoid of definite arrangement, and is not distinguishable 
into cells. This is fibrillated, and consists of strands which 
have for the most part a radial arrangement, with irregular 
vacuoles between them. In sections of specimens in which 
no food is present the appearance represented in figs. 9—11 
is fairly constant. There is always a relatively large, well- 
defined space (ent.) situated towards the middle, and repre- 
senting the intestinal lumen. The mouth usually, as in the 
specimen represented in fig. 9, leads directly into the central 
space, but sometimes a mass of the plasmodium intervenes. 
Surrounding this central space in a fairly regular manner 
are a number of smaller spaces, usually narrow and elongated, 
and having a general radiating arrangement. ‘The partitions 
between these are composed of the same material as that 
surrounding the central space, but reinforced by a good many 
muscular fibres. 
An extension of the central parenchyma in the form of a 
vacuolated horizontal plate runs out to the marginal region 
of the body on each side, and also forwards and backwards, 
and passes distally into the superficial layer. It is perforated 
by numerous dorso-ventral muscular fibres. Both above 
and below it is a series of vertically elongated spaces, like 
the cells of a honeycomb, with thin perforated walls. The 
substance of which the partitions are composed is of a more 
hyaline appearance than the central plasmodium, and is not 
fibrillated. Running in the partitions, but never through 
the spaces, are many dorso-ventral muscular fibres. The 
partitions end in the superficial layer of parenchyma. ‘lhe 
