436 
BULLETIN OF THE BUREAU OF FISHERIES. 
invaginations of the ectoderm. Some of the numerous lobes seen in figure 14 were 
formed by division of the original otocysts. Some of the lobes are completely separate 
from the rest while the cavities of others communicate with those of their neighbors 
If there is a nerve connected with the otocyst, it was not discovered. 
OSPHRADIUM. 
The osphradimn consists of a small patch of sensory epithelium situated directly 
ventral to the visceral ganglion at the origin of the branchial nerve. (Fig. 20, 0 , pi. l.) 
It is large enough to be seen with- 
out magnification and appears to be 
colorless. When examined under the 
microscope, however, its cells are seen 
to contain a yellow pigment. Nerve 
fibers are distributed to the osphra- 
dium from a ganglionic mass which 
surrounds the base of the branchial 
nerve. This nervous tissue appears 
to be a part of the visceral ganglion, 
but Pelseneer insists that the osphra- 
dium receives its nerve supply from 
the cerebro-visceral commissure and 
hence from the cerebral ganglion. I 
have no preparations to show that 
this is the case. This sense organ 
is said to be used for testing the 
purity of the water, whatever that 
may mean. 
SUMMARY. 
1. The arterial system of the 
two sides is not symmetrical, as 
may be readily seen by a com- 
parison of figures 17 and 18, which 
represent the arteries of the right 
and left sides, respectively. 
2. The venous system lacks the “sinus venosus’’ which is commonly present in 
lamellibranchs and which receives the blood from all parts of the body previous to 
entering the kidney. This sinus or a substitute for it is a necessary part of the mechanism 
described by Menegaux for extruding the foot and other organs whose movement is due 
to blood pressure. 
3. The blood in traversing the kidney passes through a closed capillary system. 
4. The blood which enters the gills must pass through a capillary system before 
emerging again. 
Fig. 15. — Drawing of transverse section of the foot showing the position 
of the otocyst. (Outlined with camera lucida.) b, Byssal gland; g, 
ventral groove in the foot; m , circular and transverse and longitudinal 
muscles; n, nerves; o, otocyst; c, ciliated tube which has given rise to 
the otocyst by invagination f rom the ectoderm; p m y undifferentiated 
mesoblast. 
