26 COMPARATIVE ANATOMY. 



cially in the head and cloacal regions. An incrustation, rusty red or l)lack, 

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

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

 red, yellowish red, lilac, or yellow in color. Echinojnenia coralliophUa, a species 

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

 pressed give the body a whitish tint resembling the tentacles of the liost, and 

 this may possibly be the case with Slrophonicuia spinosa. I^jion I'aising tlie 

 spines the pigmented hypodermis becomes less ol)scuied 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 Proneoinenia sluiteri attains, as ])reviously stated, the great 

 length of 148 mm. The average length is probal)ly not far from 30 mm. 



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

 index, or the ratio of length to breadth of body, has lieen used to a c(insiderable 

 extent, but from several ex]iei'im(Mits in the j)reser\alion of fresh material, I am 

 convinced that it is of little use, certainly not with closely I'elated forms. For 

 example nearly sixty ('huctdikrmd inonk'nuinni^i, which had come uji in the same 

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

 yet the length indices varied fully twenty i)er cent. Some si)ecimens must 

 invariably be subjected to a greater j^ressure than otiiers 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 rfjiresents a greatly reduced pedal furrow and foot. 

 In the C'haetodermatina all extei-nal tiaces of these structures have disappeared 

 completely, but internally a gap in the veiitial nuisculature 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 foinicr existence. 

 In what appears to lie the least mcidificd species, the foot consists of a single 

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

 almost ecjual height and length, and in the Necmeniidae the creeping surface 

 is often comparatively broad and is developed into se^'eral folds. Whether 

 one or more of these plaits exist each is bounded by a single la^cr of ciliated 



