§<§. 75, 76. THE ECHINODERMATA. 79 



§75. 



A peculiar calcareous plate (the madreporic plate) is observed upon the 

 cutaneous skeleton of the Asteroidea and Euhinoidea. In the last it is 

 always situated in the centre of the dorsal surface, but in the first its 

 position varies. In the proper Asteroidae there are often several, having 

 an excentric dorsal situation ; while in the Ophiuridae'^' it is found upon 

 the ventral surfiice, and especially in the angle formed by the junction of 

 the two arms with the tortuous mouth. In some Asteroidae a membra- 

 nous sac (stony canal), filled with organized calcareous particles, is attached 

 to this plate ; in others, an articulated calcareous cord stretches obliquely 

 across the body towards the border of the mouth. The use of these parts 

 is not yet positively known.'-' 



CHAPTER II. 



MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION- 



§76. 



In the Echinoderms the muscular system is well developed. Its primi- 

 tive fibres are flat, and without transverse striae.*^' 



In the ventral surface, and between each joint of the arms and pin- 

 nulae of the Crinoi'dea, there are one or two small muscles, antagonistic to 

 which, upon the opposite surface, is an interarticular elastic tissue. '-> 



In the Asteroidea, the interarticular lacunae of the internal skeleton are 

 filled with muscles.'"' The skin of these animals does not aid the motions 

 of the arms, except by its elasticity. But in the Echinidae the skin 



Duvernaea. The similar hooks found in the sea- 1S34, p. 5S0) has shown that they do not contain or- 

 mud of Vera Cruz liave been taken by Ehrenher^ diuary calcareous matter, but rather that which is 

 for stony concretions belonguig to a sponge, and organized and perforated in a reticulated manner, 

 figured and numed. Spongolithis anchora (Ab- A calcareous cord of a special structure is found in 

 hand. d. Berl. Akad. ISil, p. 323, Taf. III. No. A/<teracantkion ; see Hiebold, Mulhr's Arch. 

 VII. fig. 36). He has also taken the perforated 1836, p. 291, Taf. X. fig. 14-18 ; and Sharpey, 

 supports of these hooks for an Infusorium with a Cyclop. Anat. &c. loc. cit. II. p. 35, fig. 12, 13, s^* 

 siliceous carapace, described as Victyocha splen- 1 According to Wagner {MuUer''s Arch. 1835, 

 dens (Ibid. fig. 35). But, more lately, he has p. 319), the Echinoderms do not have transversely 

 perceived their true nature (Ibid. p. 407^ 443). striat'/d muscles. This has been confirmed by 

 The discovery of analogous cutaneous organs in the Muller (Abhand. d. Berl. Akad. loc. cit. p. 214, Taf. 

 marl near Sl:reitberg,"by Count Miinster (Beilr. IV. fig. 9) in the genera Pentacrinits and Covia- 

 z. Petrefak. lift. VI. 1843, p. 92, 96, Taf. IV. fig. tula. For my own part, I have failed to perceive 

 9), is very interesting, shice it shows the autedilu- them in Echinus, Asterias, Ophiitrus, Holotliu- 

 vian existence of Sijnapta. ria, and Hipunnulus. Valentin (Monogr. loc. cit. 

 Beside the cutaneous corpuscles of carbonate of p. 101, PI. VIII. fig. 153-155) asserts to have 

 lime, Q!/a«re/(Tire« (loc. cit. p. 36, PI. III. fig. 15) seen strise upon the fibres of the masticatory, spin- 

 has found others which are of a spherical form in ous and anal muscles of Echinus ; and C^uatre- 

 the skin of Synapta Duvernaea ; and, as they fages (Ann. de. Sc. Nat. loc. cit. p. 43, PI. IIL 

 have protractile filaments, he compares them to fig. 17) has observed transverse wrinkles during the 

 nettling organs. contraction of the longitudinal muscles of Synapta. 



1 Astrophyton. 2 Muller, Abhand. d. Berl. Akad. loc. cit. p. 214, 



2 These parts are found in Astropeeten. Ac- 220, Taf. 11. fig. 8, 12. 



corduig to Tiedemann (loc. cit. p. 54), they furnifh 3 The interarticular muscular layer of ,the 

 the necessary calcareous matter for the skeleton of Asteroidae has been accui-ately described by 

 the Asteroidae. But Ehrenberg {Multer's Arch. Meckel (System d. vergleich. Anat. III. p. 14). 



* [ ^ 75, note 2.] See, for further detaUs on this stone-canal with the Ophiuridae, Mailer, Arch. 1S50, 

 p. 122. — Ed. 



