54 ILLINOIS BIOLOGICAL MONOGRAPHS [248 



since there is almost no elongation possible and the animal, when moving, is 

 always at its greatest length. All the changes in shape are more marked in the 

 posterior middle part than anywhere else, the head almost constantly keeping 

 its form. The term 'head' is used since the anterior third of the body is con- 

 spicuously divided from the rest by a somewhat narrower neck-like portion. 

 The head, itself, is rather sharply pointed in front, the angle made being about 

 forty-five degrees. A little farther back, on each side, the outline angles again 

 forming two rather knob-like protuberances which are just a Uttle in front of 

 the eyes. Back of the eyes the neck constriction begins. This is merely a 

 curving in of the body wall, which soon begins gradually to bend outward 

 again. As far back as the middle line (a point immediately posterior to the 

 pharynx) the largest diameter is reached, giving an appearance of general 

 plumpness. The posterior one-fourth narrows back rapidly to a shghtly 

 blunt point. Altogether, the shape is rather short and broad, with a triangular 

 anterior end and a pointed posterior tip. 



The color, which has aheady been given as a delicate pink, varies from a 

 pale rose to a deep terra-cotta, but is always of surprising brilliancy. This is 

 apparent even to the naked eye, as is also the fact that it is only the posterior 

 part which bears the color, while the head region is white. Usually it is the 

 case that in most rhabdocoels there is very little if any tinge of color, since the 

 epidermis is not at all pigmented. Such a characteristic condition of the 

 integument is found here, also, and the special rose tint is lodged in another 

 tissue. As a result of metabolic processes there is produced a very clear, 

 light reddish-orange oil, which, in the shape of globules, is stored in the outside 

 layer of the digestive tract wall and, owing to the transparency of the epider- 

 mis, is distinctly visible. For this reason that part of the body in which the 

 intestine is situated is also that which is highly colored, while the anterior 

 end, into which it does not extend, remains clear. The variable condition of 

 the digestive wall as a result of m.ore or less plentiful sources of food, causes 

 very evident changes in color and also in the limits to which the color extends. 

 Consequently, when the lobes of the alimentary tract are fully extended and 

 reach far anterior, there will be some tint even in the head or, if there has been 

 little or no food taken in for some time, then the whole animal will appear very 

 light or ahnost white. Thus the color is in reality a by-product of the organic 

 processes rather than of especial intrinsic significance. 



EPITHELIUM 



The integument is made up of a single layer of high columnar cells, in some 

 portions carrying rhabdites and with a covering of ciHa on the exterior. As a 

 whole, it is an epithelium of the t5^e found generally in the Turbellaria and 

 varies only in many minor details of appearance and structure. 



These high columnar ceUs make the epidermis a conspicuous body wall, a 

 fact which is surprising on account of the extreme deHcacy of the structure. 

 Altho there is no cuticular layer, the outer boundary is very clear and distinct. 



