32 COMPAIIATIVK ANATOMY. 



ill Alexandromenia. Ami ii^aiii wlioro tlie siiiciilc retains its connection with 

 the original formative cells such a fate is out of the question. And finally a few 

 authors, notably Heuscher, have obser\('d the origin of these (jrgans directly 

 from the hypodermis. I have seen many timers papillae-like elevations (Plate 36, 

 fig. 18) such as Heuscher figures, and I am strongly of tlie opinion that they have 

 no connection with the matrix cells. 



Digestive Tract. — TIk^ anterior division of the alimentary canal consists 

 of a cavity whose walls are provided with twD folds (Mundleisten), usuafly of 

 a horseshoe-sha]ie that tlehne the cirrose area wiiere the wall is modified into 

 numerous finger-shaped filaments. In the greatiM- number of species this and 

 the succeeding jiortions of the gut are intimately united, but in Ehop. aghiD- 

 pheuiac, Dondersia, and a few other species they ai'c distinctly separated by a 

 ridge covered with the spiculose cuticle investing the body generally. Thiele 

 considers that this sensory atrium is an ectod(>rmic invagination of the integu- 

 ment corresponding ]iossibly to the ('hitdu snout. Accordingly \\here the 

 sejiaration is com]ilete the true mouth is jiosterioi' to the opening of tlie atrial 

 cavity, and where the latter is fused with th(> gut the mouth is situated behind 

 tli(^ internal buccal I'idge. In the ( "haetod(>i'matidae this anterior division is 

 absent or is represented, as Thiele assumes, by the Iniccal sensory ]ilate and 

 possibly the semicircular groove (halbmondforniigc^ (Irube). 



The limits of th(> jiharynx are difhcult to dehne but it is generally assumed 

 that it contains the radula and the outlets of the salivary glands. As a rule its 

 epithelial lining is unmoditied liumgh it ma>' be greatly folded and, in some s]ie- 

 cies form paiiillac which afford an outlet for the doi-sal sa,li\'arv glands. If an 

 oesojihagus exist it is usually not clearly diffei'entiated from the ]ihai'ynx, ami in 

 the following jiages I lia\'e disregarded it. 



In the Neomeniina tlie (ligesti\'e gland or "liver" is not differentiated from 

 the stomach or intestine. 'I'his l;ist named organ may jiossibly be represented 

 liy the ciliated tract attached to the under surface of the gonad. In the C'haeto- 

 dermatina the digestive gland, stomach, and intestine are clearly defined and 

 are not essentially different from what exists in other molluscs. The cloacal 

 chamber is probably an ectodermie invagination ami a true mantle cax'ity. 

 There is no evidence that it is an expanded rectum. 



Muscui-AU System. — This system has been described in a V(M'y few sjjecies 

 though most authors I'efer to the more ob\'ious features especially the nature 

 of the body wall. This consists';Kif an outer circular layer, resting in some spe- 

 cies u]ion a hn'er of diagonal fibrils that in turn is in contact with a system of 



