32 COMPARATIVK ANATOMY. 



in Alexandromenia. And again wlicre the spicule retains its connection with 

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

 authors, notably Heuscher, have observed the origin of these organs directly 

 from the hypoilermis. I have seen many times papillae-like elevations (Plate 36, 

 fig. 18) such as Heuscher figures, and I am strongly of the o]iinion that they have 

 no connection with the matrix cells. 



Digestive Tract. — The anterior division of the alimentary canal consists 

 of a cavity whose walls are provided with two folds (iMundleisten), usually of 

 a horseshoe-shape that define the cirrose area where the wall is modified into 

 numerous finger-shaped filaments. In the greater niunber of species this and 

 the succeeding ])ortions of the gut are intimately united, but in Rhop. aglau- 

 pheniae, Dondersia, and a few other species they are distinctly separated by a 

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

 considers that this sensory atrium is an ectodermic invagination of the integu- 

 ment corresponding possibly to the Chiton snout. Accordingly where the 

 separation is complete the true mouth is posterior to the ojiening of the atrial 

 cavity, and where the latter is fused with the gut the mf)uth is situated behind 

 the internal buccal ridge. In the Chaetodermatidae this anterior division is 

 absent or is represented, as Thiele assumes, by the buccal sensor^v plate and 

 possibly the semicircular groove (halbmondformige Grube). 



The limits of the pharynx are difficult to define but it is generally assumed 

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

 epithelial lining is unmodified though it may be greatly folded and, in some spe- 

 cies form papillae, which afTord an outlet for the dorsal salivary glands. If an 

 oesophagus exist it is usually not clearly difterentiated from the pharynx, and in 

 the following images I have disregarded it. 



In the Neomeniina the digestive gland or "liver" is not difterentiated from 

 the stomach or intestine. This last named organ may possibly l)e represented 

 by the ciliated tract attached to the under surface of the gonad. In the Chaeto- 

 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 prol)ably an ectodermic invagination and a true mantle cavity. 

 There is no evidence that it is an expanded rectum. 



Muscular System. — This system has been described in a very few species 

 though most authors refer to the more obvious features especially the nature 

 of the body wall. This consists of ah outer circular layer, resting in some spe- 

 cies upon a layer of diagonal fibres that in tui-n is in contact with a sj'stem of 



