78 THE ECHINODERMATA. ^»^^ 73, 74. 



ventral surface of the arms and pinnulae, a furrow, over which the soft 

 ekin. [perisoma) passes in a bridge-like manner. '^"^ 



§73. 



In many Echino'idea 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 masticatoi'y 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. 



The sub-cutaneous osseous ring about the oesophagus, in the Holothuri- 

 oi'dea, 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. 



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. '^^ 



§74. 



The general envelope of many Asteroidea is more or less covered with 

 Tarious calcareous productions. These have the forms of lamellae, knobs, 

 callosities, granules, immovable rays . both sharp and blunt, rough and 

 smooth movable points, double hooks, &c.'^' 



In the Echinoi'dea, 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.'-* 



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.'''* 



lO'In the Crinoidea, as well as in the Echino- bristled points, project from the surface of So- 



derms generally, the parts of the skeleton have a laster and Chaetaster. With Ophiocoma and 



calcareous, reticulated structure ; see Mii/ter's Ophiomastix, the margins of the arms are covered 



Arch. 1837, p. 93, and Ueher d. Bau. d. Pentacri- with smooth points, which in Ophiothrix are 



nus caput Medusae, in the Abhandl. d. Berl. Akad. spinous. In Ophionyx these spinous points have 



1841, Taf. I. tig. 3. movable double hooks ; see the beautiful figures 



1 Echinus, Cidaris. of Müller and Troschel (System d. Asteriden). 



2 Valentin, Monogr. Inc. cit. PI. IT. fig. 15. ^ The spines of the Echino'idea have, over their 

 ^ Agassiz, Monogr. d'Echinodermes, 21e Livr. whole extent, numerous, denticulated ribs; see 



containing the Scntellal, PI. XIII. fig. 3, I'l. Valentin, Monogr. loc. cit. PI. III. fig. '26. In 



XX.VII. fig. 7 {Lobupkora and Echinocyamiis). iSpataii^us the spines are spatulate, and in the 



* Tiedemann, \\vAt. d. Röhrenholothurie, &c., ("lypeastridae {Mellita, Encope, La^anum) they 



p. 26, Taf. II. tig. 5; also Wag'ner, Icon. zoot. are clavate. The minute researches of Valentin 



Tab. XXXII. tig. 15. (Monogr. loc. cit. p. 24, PI. Ill) have shown the 



Koren has obsorved that the osseous ring is structure of the spines of the EcliinoKlea to be very 



composed of ten jjieces witli Thyone fiiscus and complex. 



Cuvieria sr/iiamata of the Ilolothurinae. '■i The burr-like roughness of the skin of Synapta 



5 (^uatrefai^es, Ann. d. So. Nat. XVII. 1842, has already been observed by Eschscholtz (Zool. 



p. 47, PI. IV. tig. 5 ; PI. v. fig. 7, c. c. Atlas, lift. 2, 1829, p. 12). Jaei;er (De Ilolothu- 



1 With Oreaster and Culcita, the whole body is riis dissertatio, 1833, Tab. I. fig. 3) first figured 



covered with knobs and granulations. With Ax- the cutaneous books of Äy/ifl^j^a ßf.?e/i7. Quatre- 



tropecten and Utellaster, you lind flattened points Jngen (Aim d. Sc. Nat. XVII. p. 33, I'l. III.) has 



«ud marginal lamellae, innumerable rays, with given a very exact description of those of Synapta 



