474 COMPARATIVE ANATOMY chap, viii 



4. Five pairs of dorsoventral muscles connect the first five pairs of ambulacral 

 plates with the dorsal wall of the disc. By the contraction of these, the apical wall 

 of the disc is approximated to its oral wall (Fig. 381, 10). 



D. Ophiuroidea. 



A dermomusciilar tube is altogether wanting. The muscles which 

 move the brachial skeleton (the intervertebral muscles) have already 

 been described, p. 357. 



The musculature of the oral skeleton (c/. Figs. 314 and 386, pp. 359 and 486). 



1. A musculus interradialis externus connects transversely the opposite distal 

 surfaces of the oral-angle plates of neighbouring arms. This is the most powerful 

 of the muscles. 



2 and 3. The two oral-angle plates of one and the 'same arm are connected 

 by an upper and a lower transverse muscle (musculus radialis superior et inferior), 

 and approximated by means of their contraction. 



The three muscles just described form an outer circle, which is followed, orally, 

 by a second inner circle, consisting of the following muscles : — 



4. A musculus interradialis internus inferior connects transversely tiie oral 

 ends of the oral-angle plates of the different arms. 



5. The innermost muscles of the oral skeleton consist of fibres which radiate out- 

 wards. They run as five interradially placed pairs of muscles, from the oral-angle 

 plates to the teeth (in Ophiactis only to the upper teeth), for whose movement they 

 serve. These are the musculi interradiales interni superiores. 



E. Crinoidea. 



A dermomuscular tube is wanting. The musculature which moves 

 the jointed skeleton has already been described, p. 376. 



XIV. The Alimentary Canal. 



A. General Review. 



The alimentary canal, which runs through the body cavity, being 

 attached or suspended to the body wall in various ways by means of 

 mesenteries or mesenterial filaments, commences with the mouth and 

 ends with the anus. 



The absence of the anus in the Ophiuwhlea and in the Asteroid 

 family Asiriipedinidce cannot be regarded as an original condition. 



In no ease does the alimentary canal run as a straight tube from 

 mouth to anus, although, in many Synaptidm, its condition is almost 

 as simple. As a rule, the secreting and resorbing surface of the canal 

 is increased in one of two ways : — 



1. The alimentary canal, between the mouth and anus, becomes 

 increasingly lengthened, and thus necessarily forms loops, and has a 

 winding course (HoInthurioiiJea, Echinoidea, Crinoidea). 



