86 THE ECHINODBRMATA. § 85. 



smaller. Of these, five are elongated quadrilateral plates, placed at the 

 base of the lantern, between each two pyramids. The other five, smaller 

 and longer, are curved upon the first. 



All these pieces are united by many tendons and muscles to each other, 

 and to the neighboring osseous circle which projects inwards from the shell. 



The muscles of mastication are in ten pairs ; five of these arise from the 

 longest prpcesses of the osseous circle, and are inserted on the pyramids 

 below the summit of the lantern. The other five, on the other hand, pass 

 from the shortest processes of this circle to the base of the pyramids. 



By this arrangement, when the first five contract and separate the sum- 

 mits of the pyramids together with their teeth, the second five, contracting 

 also, carry the points of the teeth again together, by separating the bases 

 of the pyramids.'^' 



In the Clypeastridae, the masticatory apparatus is more simple. It is 

 composed of ten unequal, triangular pieces, joined together, V-form, two 

 and two. Each of these pieces has in its projecting angle, a furrow in 

 which a tooth is fitted. These five jaws are so arranged around the mouth 

 that their angles and the points of their teeth meet together in its centre.*"^ 



§ 85. 



The digestive cavity of the Ophiuridae is only a simple stomachal sac, 

 occupying the centre of the hollow disc of their body. 



It is divided by walls projecting inwardly, into many caeca, which never 

 extend into the rays.'^^ 



There are usually ten of these caeca, which in Astrophyton are subdivided 

 into numerous smaller caeca. ^-> 



With the Asteroidae, the stomach is large and has a similar situation ; 

 but it sends off radial caeca into the rays. 



In those species which have an anus, the digestive canal may be divided 

 into three parts. The stomach is separated into two chambers by a circu- 

 lar, projecting fold. The first of these is the true stomach, and the second 

 sends oft' the radial caeca. A narrow, short rectum, passing oft" from 

 the stomach, forms the third part of this canal, and terminates in an 

 anus, situated upon the back of the animal and concealed among points, 

 callosities, &c. This rectum has folds which, of a variable length and some- 

 times branched, are called the inter-radial caeca, and are situated between 

 instead of in the rays.'"' 



In the Comatulinae, this canal consists of a coecum situated at the end 

 of a short oesophagus, and which, after a spiral course about the axis of 

 the body, terminates in an anus having the form of a short tube projecting 

 from the ventral surface not far from the mouth. '^' 



In Comatula europaea, the axis, around which the digestive canal passes 



1 This apparatus has been minutely described by 2 Meckel, Syst. d. vergleich, Anat. IV. p. 50. 

 Tiedemann (loc. cit. p. 72, Taf. X. fig. 1, 2), hy 3 See also Tiedeniann (loc. cit. Taf. VII.), whose 

 Meckel (Syst. d. vergleich, Anat. IV. p. 56), and beautiful figures have been copied everywhere ; 

 by Valentin (Monogr. &c. p. 63, PI. V.). See and the original designs of the digestive cavity of 

 also the beautiful figure by Rymer Jones {Out- Asteracanthion,Arckaster,!aiACulcita,byMul- 

 lineof the Anim. King. &c. p. 167, fig. 70, 71). ler and Troschel (loc. cit. Taf. XI. XII.). 



2 A^assiz, Monogr. &c. 2= Livr. Scutelles. i). 4 Upon the digestive canal of Comatula, see 

 15, PI. XII. XIII. XTV. &c. Heusinger, Zeitsclir. f. d. organische Physik. III. 



1 Konrad, De Asteriarum fabrica, fig. 5. 1829, p. 371, Taf. X. XI. 



