570 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
placed muscle bands radiating from the siphons and extending toward 
the attached end of the body. This layer is best developed anteriorly, 
and is overlaid by a layer of slenderer but also closely placed bands 
encircling the body. 
Tentacles of at least four orders, the larger ones moderately 
numerous and placed with considerable regularity. 
Dorsal tubercle C- 
shaped with inrolled 
horns; the open interval 
directed forward. 
Dorsal lamina plain 
but often thrown into 
lateral undulations. 
Branchial sac with 
four well developed folds 
on each side, of which 
the three first are about 
equally high and bear 
20 to 24 internal longi- 
tudinal vessels, while the 
fourth is lower and has 
fewer vessels. Folds 
sharply defined, arising abruptly from the interspaces, the two leaves 
of each fold being closely pressed together. Interspaces between 
folds generally wide, with numerous (six to nine) internal longitudinal 
vessels. The following scheme shows about the average condition 
in the specimens studied: 
mdr. 5 to 6 (20) 7 (20) 7 (22) 8 (15) 5 to 6 en. 
Six to eight stigmata, occasionally nine, usually intervene between the 
internal longitudinal vessels on the interspaces. Transverse vessels 
of four, in some parts of the sac apparently of five orders; the smallest 
merely cross the stigmata without interrupting them. 
Stomach large, elongated, somewhat curved and abruptly distin- 
guished from the beginning of the intestine. Its walls are thin, with 
30 to 40 narrow but distinct longitudinal folds. Intestinal loop large 
but narrow, the middle part of the intestine extending down (pos- 
teriorly) so as to lie beside the stomach. Margin of anus irregularly 
lobed. 
Gonads compact, hermaphroditic, flask-shaped or elongate, and 
Text-flg. 36. — Tethyum plicatum (Lesueur). X2.2. 
