80 $77. 
covering the shell has distinct muscular bands for the motions of the 
points. 
In the Holothurioidea and Sipunculoidea there is a very thick sub- 
cutaneous muscular layer. This is itself composed of two layers, — the first. 
and upper being made up of circular, the second and lower of longitudi- 
nal fibres. In the Holothurioidea,® these fibres form five large, thick,. 
widely-spread bundles, which are inserted into the osseous ring. In the. 
Sipunculoidea, these bundles are more numerous, but more compactly 
bound together. 
The muscles of mastication, of the digestive canal, and of the tentacles, 
will be treated hereafter. 
THE ECHINODERMATA. 
§ 77. 
With the exception of the Synaptinae and Sipunculoidea, the Echinoderms 
have special, tentacular, locomotive organs (ambulacra), These are hollow 
and very contractile prolongations of the skin, and communicate through 
the ambulacral pores with small contractile sacs (ambulacral vesicles), found 
upon the internal surface of the coriaceous or calcareous envelope of the 
body. The ambulacra and their vesicles have transverse, longitudinal fibres, 
and contain a clear liquid, which, from contractions, oscillates from one to 
the other through the pores. In this way the ambulacra are capable of 
erection and elongation, and the animal uses them as feelers to find a 
proper object of attachment; and on this account, also, they have in some 
species a suctorial extremity. 
These organs, which are sometimes locomotive, sometimes prehensile, 
have the following variations of structure and form : 
I. With the Crinoidea they are small, delicate and cylindrical, and are 
found upon the borders of a furrow, which runs from the mouth along the 
soft perisoma covering the arms and pinnulae. Lach one of them is cov- 
ered with small cylindrical, clavate tentacles. 
II. The Ophiuridae have upon their arms, and between the plates, pores 
which connect with small cylindrical ambulacra; these last, from numerous 
small warts, present a studded aspect. 
III. With the Asteroidae they are situated in a double or quadruple 
row, in the ventral furrows which extend from the mouth to the end of the 
rays. They form compact cylinders of considerable size, the acute or 
truncated extremity of each of which has a sucker. 
IV. With the Echinoidea they are situated upon an elongated stalk, 
and have a sucker. They are found both upon the ambulacral plates and 
immediately around the mouth,“ 
e 4 Valentin, Monogr. loc. cit. p. 35, Pl. III. fig. 
9. 
5 The system of Holothuria 
has been described by Tiedemann (loc. cit. p. 27, 
Taf. IL. IV.); and that of Synapta by Quatre- 
Sages (Ann. d. Sc. Nat. loc. cit. p. 41). 
6 For the muscular system of Sipunculus nu- 
dus, see Grube, in Miller's Arch. 1837, p. 240, 
Taf. XI. fig. 1. 
1 The ambulacra of Comatula, which have ae- 
tive vermicul: , have no opening at 
their free extremity ; see Miller, Abhand. d. Berl. 
Akad. loc. cit. p. 222, Tab. IV. fig. 13, 14. 
2 By these the very active arms of the Ophiuri- 
Being extremely movable, they are 
dae are attached to surrounding objects; see 
Erdl in Wiegmann’s Arch. 1842, I. p. 58, Taf. 
II. fig. 1, a. 
8 Beside the very correct description given of. 
these organs by Tiedemann (loc. cit. p. 56), see 
Rymer Jones (A Gen. Outl. of the Anim. King. 
p. 148, fig. 65). It appears that in Astropecten 
the extremity of the ambulacra can be inverted, 
thus compensating for the sucker found in Echi- 
naster, Asteriscus, and Asteracanthion. 
4 With Echinus the suckers, which exactly ree 
semble the other ambulacra, are fixed upon the 
contractile membrane surrounding the mouth. 
With Spatangus and Echinanthus, there is 
