382 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
length when fairly well expanded, and often have club-shaped proc- 
esses apparently very similar to those of Lissoclinum aurcum described 
below (p. 392), but not so well developed as in that species. 
The mantle contains a considerable number of slender longitudinal 
muscle bands, but few transverse fibers. The sphincters, especially 
that of the atrial orifice, are weak. There are six rounded lobes to 
the branchial orifice; the atrial orifice is 
large and is provided with a rather short 
narrow club-shaped languet arising from 
its anterior lip. 
The oral tentacles number 16, eight of 
about the same size alternating with eight 
smaller ones. The branchial sac has four 
rows of stigmata, but the number in a 
row is not constant in different speci- 
mens. The number on each side was 
(beginning with the anterior row) 7, 8, 
8, 6 or 7, in one colony. In other speci- 
mens the maximum number was greater, 
10 or 1 1 in the middle rows. On each 
side of the endostyle is a narrow space 
without stigmata; the most ventral stigma 
of each row is small, the next larger, the 
third usually practically the full size. 
Sometimes the stigma next to the median 
dorsal vessel is also rather small. The muscles of the branchial sac 
are poorly developed. A narrow band may be observed along each 
side of the median dorsal vessel and its fibers may be traced back a 
short distance into the constricted portion of the body connecting the 
thorax and abdomen. The dorsal languets arise from the transverse 
vessels a little way to the left of the median dorsal vessel. The 
stomach is round and smooth-walled, as usual in this group. 
The male reproductive organs consist of two pyriform testes, form- 
ing together a conical mass about which the sperm duct makes from 
four to eight loose spiral turns. The ovary is elongated, lying beside 
the ascending portion of the sperm duct. The eggs in the posterior 
portion of the ovary are the furthest advanced in development. 
According to Hartmeyer fl903) and Redikorzew (1907a) this species 
has been found about Spitzbergen, the King Charles Islands, and 
Text-fig. 15. — Tetradidemnum 
alhidum (Verrill). Zooid. X 45. 
