§ 72. THE ECHINODERMATA. 77 



I. In the Holothurioklea, irregular calcareous corpuscles, which often 

 have reticulated openings, are scattered through the skin/^> 



II. In the Echinoi'dea, the calcareous substance is separated from the soft 

 skin, and composed of plates of a definite form, pierced by openings. 

 These plates are immovably united together by means of sutures. These 

 last are easily seen in the Echinidae, but are indistinct in the Clypeastri- 

 dae; they entirely disappear with age in some species.^-* Among these 

 plates which are arranged in a regular series, those called avibulacral 

 should be mentioned ; these are perforated, having upon their outer sur- 

 face the pedicles, and upon their inner the ambulacral vesicles. They 

 form, usually, five double rows, so placed between the other plates that 

 their openings form, sometimes five longitudinal rows extending from the 

 mouth to the arm,''^ sometimes a rosette of five lobes ^*> on the dorsal sur- 

 face of the skeleton.'^ 



III. The coriaceous skin of the Asteroi'dea, like that of the Holothurioi- 

 dea, contains numerous calcareous corpuscles, of which the smallest are 

 irregular, the largest porous. But beneath this is a cutaneous skeleton, 

 composed of porous calcareous pieces, movably articulated, and extending 

 on the ventral surface from the mouth to the end of the rays. 



In many species, the lai-ger corpuscles, pressed together, form a reticu- 

 lated support, which is either simple^*^ or composed of plates.^' 



With the internal skeleton, each articulation is usually composed of 

 many pieces, the intervening lacunae of which are the ambulacral fores. 

 The principal middle pieces unite at an obtuse angle, thus forming an 

 abdominal furrow.'* The Ophiuridae have also an articulated internal 

 skeleton, but the articulations are simple. But the external envelope of 

 their arms consists of calcareous scales, closely knit together, and which so 

 tightly close up the internal skeleton that the cavity of the body does not 

 extend between the skin and the internal skeleton into these appendages, 

 as in the Asteroidae. 



IV. In the Crinoidea, the skin is soft only on the ventral surface ; that 

 of the back is wholly calcareous, and converted into an articulated skele- 

 ton, which extends upon the arms and lateral branches. The mobility of 

 these articulations is due to an elastic, interarticular tissue. They consti- 

 tute discs or short cylinders, which, joined together, form arms, lateral 

 branches {pinmdae), cirri, and in some species a peduncle. '^^ 



There is a canal in axis of all these parts of the skeleton, and upon the 



1 These irregular and usually perforated calca- 2 Scutella and Clypeaster. 



reous corpuscles are mixed with the sand of the 3 Echinus and Cidaris. 



sea, after the death and decomposition of the ani- 4 Encope, Rotula, Scutella, tfC. 



mal, but can then easily be distinguished with the 5 A very detailed description of the shell of 



microscope, ^uatrefa^es (Ann. d. So. Nat. Echinus will be found in Meckel's System der 



XVn. 1842, PI. ni. IV.) has figured many of them vergleich. Anat. 11. Abth. 1, 1824, p. 31 ; and in 



belonging to Synapta. Similar microscopic cor- the monograph of Valentin, Anat. du genre 



puscles, of various forms, are found in the soft Echinus, 1841, p. 5. He has also published very 



f)art3 of most of the Eohinoderms. It is very exact researches, with figures, upon the intimate 



desirable that, as has already been commenced by structure of the calcareous plates of this animal 



Ehrenberg (Abhandl. d. Berl. Akad. 1841, p. 408), (Ibid. p. 17, PI. II.). 



they should be subjected to careful investigation ; 6 Asteracantkion, Solaster. 



for by this way alone can correct views be ob- 7 Asteracanthion, Orcaster, Solaster, tfC. 



tained upon many enigmatical- bodies of this kind 8 See the fitrure by Sharpeij, Cyclop. Anat. and 



seen by the naturalist. Phys. loc. cit. p. 31, fig. 8, 9 ; anil Meckel's ver- 



For the calcareous corpuscles imbedded in the gleich. Anat. n. Abth. 1, p. 19. 



skin of the Holothurinae, see Koren in Froriep's 9 Pentacrinus. 

 neue Not. XXXV. p. 18, fig. 6-9 ; and in the 

 Arch, skandin. Beitr. f. Natui-g. I. p. 449. 



1* 



