78 $$ 78, T4. 
ventral surface of the arms and pinnulae, a furrow, over which the soft 
skin (perisoma) passes in a bridge-like manner. 
§ 73. 
In many Echinoidea the buccal cavity is provided with processes point- 
ing perpendicularly into the interior of the shell, and which are the 
points of attachment of the masticatory muscles and ligaments. 
This osseous circle is most developed in the Echinidae.© and is com- 
posed of five processes. Between each of these is a smaller one, corre- 
sponding to as many ambulacral ones. each of which is perforated by 
a large opening.® In the Clypeastridae. there are five simple processes 
only ;® and in the Spatangidae they are wholly absent. ; oe 
The sub-cutaneous osseous ring about the cesophagus, in the Holothuri- 
oidea, corresponds probably to. this circle. Usually composed of ten pieces, 
it may be regarded as a rudimentary internal skeleton, for -it' is the point 
of attachment of both muscles and tentacles. f 8 
In Holothuria tubulosa its anterior ‘border is denticulated;® and in 
Synapta it is composed of twelve pieces, five of which have oval openings 
for. the free passage of the aquiferous canals. Tbe 
THE ECHINODERMATA. 
, 
§ 74. 
The general envelope of many Asteroidea is more or less covered with 
various calcareous productions. These have the forms of lamellae, knobs, 
callosities, granules, immovable rays both sharp and blunt, rough and 
smooth movable points, double hooks, &.? ; 
In the Echinoidea, there are points of very variable size united to 
knobs which are scattered over the ‘external surface of the shell. These 
points project through the thin skin covering this shell, having at their 
base a-kind of capsular articulation. ne et 
Remarkable cutaneous organs are found in Synapta. These are small 
anchor-like hooks,.by which these animals attach themselves to objects. 
Each of them is obliquely inserted under a small sub-cutaneous scale, 
which is perforated. by a canal. 
10 In the Crinoidea, as well as in the Echino- 
derms generally, the parts of the skeleton have a 
calcareous, reticulated - structure; see Miiller’s 
Arch. 1837, p. 93, and Ueber d. Bau. d. Pentacri- 
mus caput Medusae, in the Abhandl. d. Berl. Akad. 
1841, Taf. I. fig. 3. 
1 Kchinus, Cidaris. 
2 Valentin, Monogr. loc. cit. Pl. IT. fig. 15. 
8 Agassiz, Monogr. d’Echinodermes, 2ue Livr. 
containing the Scutedlal, Pl. XIII. fig. 3, Pl. 
XXVII. fig. 7 Lobophora and Echinocyamus). 
4 Tiedemann, Anat. d. Réhrenholothurie, &.,; 
p. 26, Taf. II. fig. 5; also: Wagner, Icon. zoot. 
Tab. XXXII. fig. 15. 
Koren has observed that the osseous ring is 
composed of ten pieces with Thyone fuscus and 
Cuvieria squamata of the Holothurinae. 
5 Quatrefages, Ann. d. Sc. Nat. XVII. 1842, 
p. 47, PL LY. fig. 5; Pl. ¥. fig. 7, 6, ©. 
1 With Oreaster and Culcita, the whole body is 
covered with knobs and granulations. With As- 
fropecten and Stellaster, you find flattened points 
and marginal lamellae. Innumerable rays, with 
bristled points, project from the surface of So- 
laster' and Chaetaster. With Ophigcoma and 
Ophiomastia, the margins of the arms are covered: 
‘with smooth points, which in Ophiothrix are 
spinous.’ In Ophionyx these spinous points -have 
movable double hooks; see the beautiful figures 
of Miller and Troschel (System 4. ‘Asteriden). 
2 The spines of the Echinoidea have, over their 
whole extent, numerous, denticulated ribs; see 
Valentin, Monogr. loc. cit. Pl. III. fig. 26.. In 
Spatangus the spines are spatulate, and in the. 
Clypeastridae (Mellita, Encope, Laganum) they 
are clavate. The minute researches of Palentin 
(Monogr. loc. cit. p. 24, Pl. IIT) have shown the 
structure of the spines of the Echinoidea to be very 
complex. se 
3 The burr-like roughness of the skin of Synapta 
has already been observed by Eschscholtz (Zool. 
Atlas, Hft. 2,'1829, p. 12). Jaeger (De Holothu- 
riis dissertatio, 1833, Tab. I. fig. 3) first. figured 
the cutaneous hooks of Synapta Beselit. Quatre- 
Sages (Aun. d. Sc. Nat. XVII. p. 33, Pl. ILL.) has 
given a very exact description of those of Synapta 
