§ 84. THE ECHINODERMATA. 85 



■which, by their contraction, is pressed into the cavity of the tentacles for 

 lubrication/-' 



The retraction of the tentacles is due in part to their own contractility, 

 and in part to the numerous muscles, which, arising from the internal sur- 

 face of the cavity of the body, are inserted into the osseous circle. By 

 these means, it, together with the tentacles, can be retracted into the 

 body.'^' With the Sipunculidae the tentacular apparatus consists of a 

 fringed border on the margin of the mouth, which is also provided 

 with vesicles.'^' In Siptoiculus, and Phascolosoma, there are fom- long 

 muscles, which, arising from the internal surface of the body, pass on to the 

 mouth, and are retractors of the tentacular membrane.*^' It is possible 

 that these oral tentacles serve not only as prehensile organs of food, but 

 also as those of locomotion and respiration.*^'* 



§ 84. 



The mouth of the Comatulinae presents nothing remarkable ; but with 

 the Asteroidae, it is covered with hard papilla;, projecting from its corners 

 and angles. In the Dphiuridas, the inverted angles are covered with hard 

 papillae, while the everted ones have calcareous teeth, between which are 

 concealed soft cylindrical tentacles. Immediately behind all of these, the 

 entrance of the stomach is indicated by a membranous sphincter. lu the 

 Asteroidae, however, this is wanting, there being a short oesophagus leading 

 directly into the stomach. 



With the Echinoidea, and Holothurioidea, the mouth has a soft circular 

 lip, between which, with the Echinidae, and with the Clypeastridae, pro- 

 ject the points of enamelled teeth. 



The mouth of the Echinidae, and Clypeastridae, has a very remarkable 

 masticatory apparatus. In the first, the calcareous basis which supports the 

 teeth has long been known as Aristotle's lantern. This conical basis is 

 divided into a base and summit ; the first being the superior part of the 

 animal itself, while the second is formed by points of teeth projecting 

 from the mouth. It is, moreover, composed of fifteen pieces, five of which 

 are three-sided, hollow pyramids, and so adjusted that they touch each 

 other by their plane sm'faces, presenting externally the third surface which 

 is convex. This last has internally a longitudinal furrow, in which is fitted a 

 very long, narrow and slightly-curved tooth. Beside these five principal 

 pieces, which form the jaws of EcJiiaus, there are two other kinds, much 



2 These vesicles are found in Holothuria and 3 in Pentacta, there are five large cylmdrical 



Chirodota ; see Tiedemann, loc. cit. Tab. 11. fig. muscles arising from the subcutaneous longitudinal 



4, e. 6, i. ; also the Catalogue of the Museum, Lon- ones, and inserted into the osseous circle ; — they 



don, &c., rV'. PI. XLIX. fig. 1, 2 {Holothuria tubu- are special retractors of the tentacles ; see il/ecAe/, 



losa) ; and the Atlas Zool. du Voyage de 1' Astrolabe. System d. vergleich, Anat. IV. p. 62. 



Zoophytes, PI. VIII. fig. 3 {Chirodota fusca). 4 I am incUned to regard as tentacular the two 



In PenJacfa do^z'o^yjn, I have found only a single vesicles of PoU, in Sipuncvlus ; and of which 



cylindrical vesicle fixed to the circle of tentacles. Delle Chiaje (Jlemor. kc. Tav. I. fig. 6, d.) per- 



In Synapta Duvernaea {Quatrefages Ann. d. ceived only one, although Grube (3/u//er's Arch. 



Sc. Nat. loc. cit.), these vesicles are entu-ely want- 1837, p. 251, Taf. XI. fig. 2, P.) has seen them 



ing. both fixed in a space circumscribed by the tentac- 



Cuvier (Anat. Comp. V. p. 454) and other anat- ular membrane, 



enlists (see Grant, Outlines, &c., p. 333) have 5 Gn^fte, Ibid, p. 241, Taf. XI. fig. l,u. 2, m.m.; 



erroneously taken these parts for saUvary organs, and Delle Chiaje, Memor. &c. Tav. I. fig. 3. 



They do not communicate with the digestive canal, 6 The oral tentacles of Synapta Duvernaea, 



but connect freely with the circulatory and respir- which, according to Quatrefages (loc. cit. p. 63, 



atory systems, — a point, therefore, to which we PI. IV. fig. 1), have suckers on their internal sur- 



shall further aUude hereafter. face, are certainly used as locomotive organs. 



8 



