654 DAVID HILT TENNENT. 
The body continually changes in shape, stretching out so 
that it becomes long and narrow, or suddenly contracting so 
that it is but slightly longer than broad. The tails, when 
fully extended, may reach a length of from five to ten mm. ; 
when contracted they are about the length of the body. 
As seen in transmitted light, the body is nearly colorless, 
and, with the exception of opaque masses of cells which are 
the rudiments of various organs, is perfectly transparent. 
Through the clear integument may be seen the contractile 
sac of the water-vascular system, the pharynx and gut, the 
rudiments of the penis sheath and the testes, the cystogenous 
organ, and many unicellular glands (giant cells, sunken 
epithelial cells) (fig. 1). 
In swimming the posterior end of the body is directed 
forwards, the tails rapidly lashing from above downwards. 
When the creature is examined under a cover-glass the 
tails are moved slowly back and forth or twisted into intricate 
knots, ultimately being thrown off. With the loss of the 
tails, however, locomotion does not cease. By means of peri- 
staltic contractions, passing from behind forwards, the creature 
is able to creep slowly forward. During these motions the 
anterior end is pushed out and broadened so that it assumes 
a disc-like appearance, while the cystogenous organ is pushed 
forward and constricted. 
Tur CUTICLE. 
The entire animal is covered by a transparent cuticle 
(“ Hautschicht,” of the older German investigators) from 
0-002 mm. to 0:0025 mm. in thickness, which in surface views 
appears to be divided into rectangular areas of uniform size 
(fig. 31). This appearance has been described in B. poly- 
morphus by Stewart (32) and by Ziegler (7). It is to this 
peculiar pavement-like structure of the cuticle that the 
striated appearance of the body, described by several ob- 
servers, is due. The mosaic effect is caused, I believe, by 
the growing spines which are arranged in rows parallel to 
