^ 84. THE ECHINODEKMATA. 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 Sipuneulidae the tentacular apparatus coHsists of a» 

 frino-ed border on the margin of the mouth, which is also provided 

 with vesicles.'^' In Sipunculus, and Phascolosonia, there are four 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 papillae, projecting from its corners 

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

 papillse, 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. In the 

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

 directly into the stomach. 



With the Eahinoidea, 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 surfaces, 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 Echinus , there are two other kind^, much 



2 These vesicles are found in Holotkuria and 3 in Pentacta, there are five large cjMindrical 



Chirodola ; see Tiedemann, loc. cit. Tab. II. fig. muscles arising from the subcutaneous longitudinal 



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



don, &c., IV. I'l. XLIX. fig. 1, 2 {Holotkuria tubu- are special retractors of the tentacles ; see Meckel., 



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



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



In Pe/iiac<a rfo/(o/«?n, I have found only a single vesicles of PoU, in Sipunculus; and of which 



cylindrical vesicle fi.xed to the cu-cle of tentacles. Delle Chiaje (Memor. &c. Tav. I. fig. 6, d.) per- 



In Synapta Duvernaea (Qualrefages Ann. d. ceived only one, although Grube (Muller^s Arch. 



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



ing. both fi.xed in a space cii'Cumscribed by the tentac- 



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



omists (see Grant, Outlines, &c., p. 333) have 5 GrMftc, Ibid, p. 241, Taf. XI. fig. 1, u. 2, m. m.; 



erroneously taken these parts for salivary organs, and Del/e Chiaje, Memor. &c. Tav. I. fig. 3. 



They do not communicate with the digestive canal, G 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- 



Bhall further allude hereafter. face, are certainly used as locomotive organs. 

 o 



