40 CU.MPARATIVE ANATOMY. 



tlie process, and it has evon been suggested that the entire stomach-intestine 

 may be active. In the C'haetodermatina definite ctenidia perform the respira- 

 tf)ry functii)n, while in the Neomeniina the thin walls of the cloacal ea\ity, often 

 thrown into folds, sometimes of enormous size, and in contact with extensive 

 blood sinuses, are undoubted active agents in this respect. 



In some species, such as in Alexandromenia the atrial ridges ai-e of large 

 size and their superficial extent is increased l)y the development of secondary 

 folds and papillae. As these are penetrated by large blootl sinuses it is probable 

 that they act to a certain extent as resjjiratory organs though it is doubtful if 

 this is their chief duty. The cirri on the other hand never, or very rarely, con- 

 tain I)lood spaces and are very probably special sense organs. 



As the functions ascribed to the various sense organs, real or supposed, have 

 been tested experimentally in a few cases only, it is not surprising that the 

 opinions of authors differ widely. Considering first the sense of touch it is 

 probable that it is located over the general body surface, for living specimens of 

 Chaetoderma cruditn and C. 'inonkrcycnsis respontl to mechanical stimuli apjtlied 

 at any jioint. A numl)er of investigators have noted the presence of nerves in 

 contact with the hypodermis in other sj)ecies and Wiren has traced some of 

 them into the deeper portions of the cuticle where they are supposed to function 

 in the sense of touch. In those species with thin cuticle and freely projecting 

 spines it is likewise supposed that the latter serve as tactile organs. This same 

 activity is ascribed a'so by Thiele to sense cells which he has detected in the 

 foot of certain Neomeniidae (Ncomcnia grandis, Proncornenin ivgan^i). Of the 

 various activities which have l)cen connected with the nuich discussed papillae 

 of the hypodermis is the ability to distinguish vibrations, amount of water 

 pressure, or more generalized stinuili afl'ecting the tactile sense. It is to be 

 noted, however, that on the other hand these organs are said by several authors 

 to be strictly glandular. A remarkable problematical relation of some of the 

 papillae, considerably modified, to the anterior coecum of the stomach-intestine 

 in Halomenia gravida (p. 147) is difficult to explain on any hypothesis. 



From experiments Wiren finds the gills of Chaetoderma to be very sensitive, 

 and considers that the stiff hairs situated among the cilia are prol)ably tactile. 

 The buccal ])apillae are said by Heuscher to be organs of touch; they are cer- 

 tainly not universally, if ever, food collecting organs. In close connection with 

 the anterior border of the moutli of a few species in a living condition Pruvot, 

 and Kowalevsky and Marion have detected sensory hairs, that in some species 

 are attached to elevations, ajiparently the homologue of the sensory ridge, that 



