Marshall and Severin.—Anatomy of Ranantm fuscaL 491 
position for some time and then may descend, or, allowing the 
body to rise, occupy a position nearly parallel to the surface of 
the water. The rectum filled with air acts as a hydrostatic blad¬ 
der to carry the posterior part, of the body up first. We do not 
see that there can he any other use for the rectum. The cells 
from their structure and evident lack of protoplasm could not 
serve any active function, and the absence of any except very 
small trachae would preclude the possibility of a respiratory 
function. 
Salivary glands .—In a general dissection, dorsal view, the 
salivary glands appear as two tubes extending irregularly from 
a position near the center of the prothorax hack to the anterior 
end of the thoracic ganglion. (PL XXXIV, Fig. 1.) An exami¬ 
nation of a number of specimens shows that the glands vary as to 
their relative position to each other and to their 1 entire length, 
some ending before the thoracic ganglion is reached, and others 
extending beyond its posterior margin, or in exceptional cases, 
to near the beginning of the abdomen. At the anterior end of 
each of these glands, there is a smaller one lying, in a dorsal 
view, entirely or partially concealed. E!ach small gland appar¬ 
ently unites with the large one near its anterior end, and from 
this union a single tube passes forward. A closer examination 
shows, as Dufour (2) has figured, that wdiile each gland has its 
own duct and these join, yet they separate again and pass for¬ 
ward distinct from each other. An examination of glands, 
mounted m to to , and of sections, shows that each # one consists of 
a median, longitudinal tube passing from one end to the other 
and surrounded by a single layer of acini, all sessile and opening 
into the central duct. The shape of the acini vary from those 
in the thicker parts of the gland with a round outline, to the 
elongated ones found in its narrower parts. Eiach acinus (PL 
XXXIV, Fig. 9) consists of a large central lumen partially or 
entirely filled with a, granular secretion, and a wall composed of a 
number of flattened cells each of wdiich contains a large nucleus. 
In the sections we studied it was impossible to distinguish any 
cell boundaries; the nuclei w 7 ere very large, from one to four oc¬ 
curring in each section. Besides the cells of the smaller gland 
having a shorter diameter than those of the large one, there is 
noticeable in stained specimens a difference in color. In sec¬ 
tions stained in haematoxylin followed by eosin, the contents of 
the smaller gland is nearly colorless but in the larger gland the 
