78 



THE ECHINODERMATA. 



§§ 73, 74. 



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

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



§H70 

 io. 



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. 



The sub-cutaneous osseous ring about the oesophagus, 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. 



In Hnlothuria 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. ^^^ 



§ ■?4. 



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, &c.'^^ 



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.^-* 



Remarkable cutaneous organs are found in Sytiapta. 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- 

 derm3 geuerally, the parts of the skeleton have a 

 caicareaus, reticulated structure ; see MiUler''s 

 Arch. 18o7, p. 93, and Ueber d. Ban. d. Pentaori- 

 nu3 caput Medusae, m the Abhandl. d. Bed. Akad. 

 1841, Taf. I. fig. 3. 



1 Echinus, Cidaris. 



2 Valentin, Moiiogr. I09. cit. PI. II. fig. 15. 



3 Agassiz, Mouogr. d'jfcchiuodermes, '2 le Livr. 

 containing the Scutellal, PI. XIII. fig. 3, PI. 

 XXVIl. fig. 1 {Lobophora and Echinocyamxis). 



i Tiedemann, Anat. d. ROhrenholothurie, &c., 

 p. 26, Taf. II. fig. 5 ; also Wagner, Icon. zoot. 

 Tab. XXXII. fig. 15. 



Korea has observed that the osseous ring is 

 composed of ten pieces with Thyone fuscus and 

 Cuvieria squamata of the Holothuriuae. 



6 quatrefages, Ann. d. Sc. Nat. XVII. 1842, 

 p. 47, PI. IV. fig. 5 ; Pi. V. fig. 1, c. c. 



1 With Or easier and Culcita, the whole body is 

 covered with knobs and granulations. With As- 

 tropecten and Stellaster, you find flattened points 

 and marginal lamellae. Innumerable rays, with 



bristled points, project from the surface of So- 

 lafster and C/iactaster. With Ophiocoma and 

 Ophiomaslix, the margins of the arms are covered 

 with smooth points, which in Ophiolhrix are 

 spinous. In Ophionyx these spinous points have 

 movable double hooks ; see the beautiful figures 

 of Mailer and Troschel (System d. Asteriden). 



- The spines of the Echinoidea have, over their 

 whole extent, numerous, denticulated ribs ; see 

 Valentin, Mouogr. loc. cit. PI. III. fig. 26. In 

 Spatangus the spines are spatulate, and in the 

 Clypeastridae {Mellita, Encope, Laganum.) they 

 are clavate. The minute researches of Valentin 

 (Monogr. loc. cit. p. 24, PI. Ill) have shown the 

 structure of the spines of the Echinoidea to be very 

 complex. 



3 The burr-like roughness of the skin of Synapta 

 has already bseu observed by Eschscholtz (Zool. 

 Atlas, lift. 2, 1829, p. 12). Jaeger (De Ilolothu- 

 riis dissertatio, 1833, Tab. I. fig. 3) first figured 

 the cutaneous hooks of Synapta Beselii. Quatre- 

 fages (Ann. d. Sc. Nat. XVII. p. 33, PI. III.) has 

 given a very exact description of those of Synapta 



