154 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
one of the red secreting cells found by Frenzel (92) to be common 
in the small intestine of Synapta, Strongylocentrotus, and Holo- 
thuria tubulosa. Frenzel thinks these may represent the beaker-like 
gland cells of Hamann. I have not found special gland cells in the 
small intestine. I infer from the absence of gland cells and the 
great development of the blood sinuses connected with this part of 
the alimentary tract, that it is concerned solely with absorption. 
6. The epithelium of’ the large intestine resembles closely that 
of the part of the alimentary canal just described. The cells have 
each a similar, deeply-staining cuticular cap, but the whole tract 
lacks the extensive blood supply of the small intestine. 
The epithelium of the cloaca (Plate 5, fig. 62) is composed 
exclusively of columnar cells, whose nuclei occupy the middle or 
basal portion of the cell. In the tail region it is about 29 m thick, 
and is provided throughout with a thick cuticula. The ordinary 
spheruliferous bodies are abundant in the subjacent layer of connee- 
tive tissue. 
7. THE RESPIRATORY TREES. 
Opening into the cloaca on either side, at points a little behind 
the bend in the small intestine, are the two respiratory trees (Plate 
4, fig. 46); they are quite distinct from each other. Both trees run 
forward to the region of the aquapharyngeal bulb. 
The left tree divides near its attachment into two main branches, 
one of which follows, and is intimately connected with, the left 
perforated mesentery of the large intestine, and therefore may be 
called the ventral branch, while the other lies in the anti-mesente- 
rial blood plexus of the small intestine, and may be distinguished 
as the dorsal branch. The right tree consists of a single trunk; this 
lies partly in the meshes of the right mesentery of the large intestine, 
which it accompanies. Anteriorly it crosses on the dorsal side of 
the aquapharyngeal bulb to the left side of the body, where it 
soon ends blindly. 
Kingsley probably did not recognize the dorsal branch of the left 
tree, for he neither described nor figured it; hence he concluded that 
the right tree is the larger ; if, however, both branches of the left tree 
be taken into consideration, the left much exceeds the right in size. 
