ON OTENOPLANA. 327 
peripheral anastomosing ramifications of the gastric system, 
and have not indicated them in the sketches of the external 
form. 
When Ctenoplana wishes to sink from the surface to the 
bottom it doubles itself up in the usual way, and so sinks 
apparently without employing the combs. This was also ob- 
served by Korotneff. 
2. The Aboral Sense-organ.—As already described by 
Korotneff, the aboral sense-organ consists essentially of an 
otolithic mass, suspended by stiff processes from adjacent cells 
in a cupule, and surrounded by a ring of ciliated tentacles. 
Korotneff figures the latter in the form of a simple circlet. 
This, however, is not the case. The circlet of sensory 
tentacles surrounding the otolith consists of two 
distinct and separate halves, with about nine ten- 
tacles in each half. The one half is placed on one 
side, and the other on the opposite of the tentacle 
axis (fig. 1). 
This is perhaps the most important observation that I was able 
to make on the living animal, and it is a crucial one for deciding 
upon the homologies of the axes of Ctenoplana with those of 
bilateral animals. The division of the circlet of sensory 
tentacles into two portions was remarkably distinct and un- 
equivocal. The sensory or apical tentacles (as distinguished 
from the muscular or terminal pinnate tentacles) are usually 
carried extruded (figs. 2 and 3), but they can be completely 
retracted. 
3. Cilia.—I cannot confirm Korotneff in his statement as 
to the general distribution of cilia over the surface of the body. 
The places where I have observed cilia (apart, of course, from 
the ctenophoral plates) are as follows: (i) on the sensory 
tentacles, (11) on the cells lining the sheaths of the pinnate 
tentacles, and (iii) over a large area of ventral surface (fig. 5). 
I must deny the presence of cilia on the general dorsal 
surface. 
1 As Ctenoplana is semi-opaque, it is difficult to discern much of the 
internal structure in surface view. 
