40 COMPARATIVE ANATOMY. 



the process, and it has even been suggested that the enth-e stomach-intestine 

 may be active. In the Chaetodermatina definite ctenidia perform the respira- 

 tory function, while in the Neomeniina the thin walls of the cloaca! cavity, often 

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

 blood sinuses, are undoubted active agents in this respect. 



In some species, such as in Alexandromenia the atrial ridges are of large 

 size and their superficial extent is increased by the development of secondary 

 folds and papillae. As these are penetrated by large blood sinuses it is prot);ible 

 that they act to a certain extent as respiratory organs though it is doulitful if 

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

 tain blood 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 erudita and C. viontercyensis respond to mechanical stimuli applied 

 at any point. A number of investigators have noted the presence of nerves in 

 contact with the hypodermis in other species 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, Proneomcnia vagans). Of the 

 various activities which have been connected with the much discussed papillae 

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

 pressure, or more generalized stimuli affecting 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 probably tactile. 

 The buccal papillae 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 mouth 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, apparently the homologue of the sensory ridge, that 



