82 Papers from the Marine Biological Laboratory at Tortugas. 
There is much confusion, among writers on the embryology of Salpa, 
as to the nature and homologies of the so-called “ gill,’ which is in no way 
comparable to the gill-clefts of Doliolum and Pyrosoma and other Tunicata. 
If Salpa has gill-slits, it has only one on each side, and this is enormous, 
putting the pharynx into free communication with the cloaca or atrium. 
The so-called “ gill” is homologous with the dorsal lamella of other tuni- 
cates. It is a rod-like organ, extending obliquely upward and forward 
from the middle line of the ventral surface in the region of the cesophagus 
to the middle line of the dorsal surface near and a little posterior to the 
ganglion, thus separating the pharynx from the atrium on the middle line, 
while the lateral portions of the pharynx are in free communication with 
the lateral regions of the atrium. There is, in Salpa, no recognizable bound- 
ary between the lateral atria and the median atrium, and it has seemed best, 
for this reason, to most writers on Salpa to call the whole atrial system the 
cloaca. The “gill” is nearly circular in cross-section; its anterior or 
pharyngeal surface consists of the ciliated pharyngeal epithelium of the 
dorsal lamella, while its side walls and posterior or cloacal surface consist 
of the ectodermal epithelium of the cloaca. The “gill” is hollow, lined 
Fic. 2—The aggregated Salpa pinnata in side view. 
with mesoderm, and its chamber is part of the body-cavity. It is in this 
chamber that the intestine of the solitary Cyclosalpa lies. 
While the solitary cyclosalpas agree in this respect, the aggregated ones 
differ greatly in the form and position of the intestine. In the aggregated 
S. pinnata the intestine is long, as it is in the solitary form, and the anus 
is far forward, but the intestine is not in the gill, and it lies on the middle 
line of the ventral surface, in the body-cavity under the endostyle, as is 
shown in text-figure I. 
In all the other aggregated cyclosalpas the intestine is very different from 
that of the solitary ones, and forms a loop with the gastric end and the anal 
