§ 72. 
I. In the Holothurioidea, irregular calcareous corpuscles, which often 
have reticulated openings, are scattered through the skin. 
II. In the Echinoidea, 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 Hchinidae, 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 ambulacral 
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 Asteroidea, 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 larger 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 pores. 
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 (pinnudlae), cirri, and in some species a peduncle. 
There is a canal in axis of all these parts of the skeleton, and upon the 
THE ECHINODERMATA. 77 
1 These irregular and usually perforated calca- 
reous corpuscles are mixed with the sand of the 
sea, after the death and decomposition of the ani- 
mai, but can then easily be distinguished with the 
microscope. Quatrefages (Ann. d. Sc. Nat. 
XVII. 1842, Pl. III. IV.) has figured many of them 
belonging to Synapta. Similar microscopic cor- 
puscles, of various forms, are found in the soft 
parts of most of the Echinoderms. It is very 
desirable that, as has already been commenced by 
Ehrenberg (Abhandl. d. Berl. Akad. 1841, p. 408), 
they should be subjected to careful investigation ; 
for by this way alone can correct views be ob- 
tained upon many enigmatical bodies of this kind 
seen by the naturalist. 
For the calcereous corpuscles imbedded in the 
skin of the Holothurinae, see Koren in Froriep’s 
neue Not. XXXV. p. 18, fig. 6-93 and in the 
Arch. skandin. Beitr. f. Naturg. I. p. 449. 
7* 
2 Scutella and Clypeaster. 
3 Echinus and Cidaris. 
4 Encope, Rotula, Scutella, &c. 
5 A very detailed description of the shell of 
Echinus willbe found in Meckel’s System der 
vergleich. Anat. II. Abth. 1, 1824, p.31; and in 
the monograph of Valentin, Anat. du genre 
Echinus, 1841, p. 5. He has also published very 
exact researches, with figures, upon the intimate 
structure of the calcareous plates of this animal 
(Ibid. p. 17, Pl. IL). - 
6 Asteracanthion, Solaster. 
7 Asteracanthion, Oreaster, Solaster, &c. 
8 See the figure by Sharpey, Cyclop. Anat. and 
Phys. loc. cit. p. 31, fig. 8,9; and Meckel’s ver- 
gleich. Anat. II. Abth. 1, p. 19. 
9 Pentacrinus. 
