STUDIES ON THE TURBELLARIA. 427 
except that the head end is very frequently raised, sometimes 
in mid-water, by the agency of the cilia. When the movement 
is more rapid it is brought about by rhythmical undulations, 
in which the thin lateral flanges and the pointed anterior 
regions take the leading part; by means of similar movements 
the animal is able to swim rapidly in mid-water. As the 
animal moves along the anterior region is turned about 
actively in all directions, probing and testing, and seems to 
be by far the most sensitive part, responding with great 
rapidity to contact with foreign bodies—contact wth other 
individuals of the same species causing a particularly active 
recoil often followed by what appears like a rapid aggressive 
movement. When at rest the anterior and lateral parts, 
together with the posterior processes, alone adhere to the 
surface, the body being somewhat arched in such a way that 
there is a wide space below into which the mouth and repro- 
ductive apertures open—the space communicating with the 
exterior freely behind between the posterior processes. 
There is no frontal organ, such as occurs in most other 
Acela.! 
The mouth, a short longitudinal slit, is situated towards 
the middle of the ventral surface. Behind it are the two 
median reproductive apertures, the female in front and the 
male behind, separated from one another by a space which is 
considerably less than that separating the anterior from the 
mouth, the posterior separated from the posterior border by 
an interval slightly longer than that which intervenes between 
the female aperture and the mouth. There is a pair of minute 
eyes towards the anterior end, and an otocyst situated 
between them. In a mature animal the vesicula seminalis 
projects as a rounded prominence on the dorsal surface near 
the posterior border, and in front of this, also on the dorsal 
surface, is a well-marked depression marking the position of 
the aperture of the supposed Laurer’s canal, 
The colour, which varies greatly, depends partly on the 
symbiotic Algz present, partly on pigment in the epidermis. 
1 Not in Haplodiscus ussowii, according to Sabussow (24). 
