COMPARATIVE ANATOINIY. 27 



epithelium which as a rule is continuous with the lining of the cloaca. The 

 last named space is certainly a true mantle cavity and the plume-like branchia 

 it contains in the Chaetodermatidae are ctenidia. On the other hand it is 

 questionable if the folds developed in the cloacal walls, as in Alexandromenia 

 for example, are homologous organs. 



As in the Chiton embryo two sets of pedal glands exist, termed by Hubrecht 

 ('80) the anterior and posterior pedal. The first named is a highly developed 

 organ filling the greater part of the space between the body wall and gut in the 

 head region. It is composed of pyriform cells whose ductules lead into the 

 anterior end of the pedal furrow, which is usually developed into a cavity of 

 considerable size, where they make their exit by separate intercellular openings. 

 The i)osterior pedal gland is situated above and slightly to each side of the foot 

 throughout its entire extent. Its cells are likewise pear shaped and open inter- 

 cellularly into the pedal furrow. 



Hypodermis and Products. — What is sometimes termed the skin consists 

 of two main elements, the hypodermal cell layer and the o^•erlying spiculose 

 cuticle. In the majority of species the hypodermis consists of a single layer of 

 cells, and exceptionally {Paramenia palifera, Ichlhijomenia ichthyodes, for exam- 

 ple) forming a more or less irregular many cell layer. Concerning the nature 

 of the elements entering into its formation there are numerous differences judg- 

 ing from the accounts of the various authors, and the functions ascribed to them 

 are equally diverse. The ordinary hypodermal cells, those responsible for the 

 formation of the cuticle, are usually cubical or low columnar in form with round 

 or oval nuclei imbedded in a finely granular cytoplasm pigmented in a few spe- 

 cies {e. g. C. nitidulum) which blends with the overlying cuticle. In the same 

 general situation gland cells are present in several species together with more 

 slender elements which may perform a sensory function. 



The papillae occur in all Neomeniina in which there is more than one layer 

 of spicules imbedded in the cuticle. Generally speaking each consists of a 

 comparatively slender stalk which is attached to the hypodermis, and on the 

 other hand expands into a more or less globular mass in contact with the free 

 surface of the cuticle or may even project above it. The cells composing the 

 swollen portion are apparently filled in life with a highly spongy, possibl>- vacuo- 

 lated protoplasm which in preserved material may shrink greatly, producing 

 radiating pseudopodia-like processes. In Halomenia gravida outpouchings of 

 the gut occur at fairly regular intervals along the dorsal side of the animal on 

 each side of the mid line. These penetrate the somatic musculature (Plate 32, 



