26 COMPARATIVE ANATOMY. 



cially in the head and cloacal regions. An incrustation, rusty red or black, 

 may cover the entire animal though it is usually restricted to the posterior 

 extremity. In a very few species some of the hypodermal cells contain pigment, 

 red, yellowish red, lilac, or yellow in color. Echinomenia coralliophila, a species 

 living on CoraUium ruhrum, is provided with movable scales which when de- 

 pressed give the bod\' a whitish tint resembling the tentacles of the host, and 

 this may possibly be the case with Strophomenia spinosa. Upon raising the 

 spines the pigmented hypodermis becomes less obscured and the animal assumes 

 a reddish color similar to the coral stalk. 



The smallest sexually mature Solenogastres are not over 5 mm. long, and 

 on the other hand Proneomenia sluiteri attains, as previously stated, the great 

 length of 148 mm. The average length is probably not far from 30 nnn. 



Length Index. — In the discrimination of species the so-called length 

 index, or the ratio of length to breadth of body, has been used to a considerable 

 extent, but from several experiments in the preservation of fresh material, I am 

 convinced that it is of little use, certainl}' not with closelj' related forms. For 

 example nearly sixty Chaetoderma montereyensis, which had come up in the same 

 dredge haul, were treated with slow alcohol in precisely the same fashion and 

 yet the length indices varied fully twenty per cent. Some specimens must 

 invariably be subjected to a greater pressure than others in the dredge load, 

 and these are more flaccid and less contractile and with them the length index 

 is relatively greater. 



COMPARATIVE ANATOMY. 



Foot and Glands. — It is now a generally accepted fact that the ventral 

 furrow and its included fold represents a greatly reduced pedal furrow anil foot. 

 In the Chaetodermatina all external traces of these structures have disappeared 

 completely, but internally a gap in the ventral musculature and a thickening 

 of the longitudinal muscles on each side of the mid ventral line and in Limifossor 

 a well-developed pedal sinus in the head region indicate their former existence. 

 In what appears to be the least modified species, the foot consists of a single 

 fold, but in several other species this is accompanied on each side by a fold of 

 almost equal height and length, and in the Necmeniidae the creeping sui'face 

 is often comparatively broad and is developed into several folds. Whether 

 one or more of these plaits exist each is bounded by a single layer of ciliated 



