HAD I ATA 



693 



The Pe-ntacrimis, however, must not he regardea aS the true i^pe of Die C'l-inriid order, hut r;itbor as a link of 

 li an^itiall which connects it with the higher furms of Echinodermata. For the- hulk of the group is made up of 

 the very numerous tribe of EvcriniUs, which seems to have been in many respects of lower 

 organization, connecting the free Echinoilermat;! with Zoophytes. The body and jointed 

 stem exhibit a rounded instead of a pentagonal form ; the latter is usually destitute of se- 

 condary arms ; and the principal branches do not ramify with the same minuteness as tho,se 

 of many Pentacrini. The stalk seems to have been attached by a f ort of spreading root, re- 

 sembling that of many Corals ; and we must therefore believe this tribe of Crinoidete to have 

 been entirely fixed. It contains a numerous series of forms ; some of them ahriost re- 

 sembling stalked Echini, whilst others in like manner seem to connect the order with the 

 twn following. 



II. Opiiiukid^e. These Star-fishes are so named frnm the long serpent- or worm-like arms, 

 wliieh are appended to their round, depressed, urcliin-like bodies. Although commonly as- 

 sociated w th the true Star-fish, they are very distinct in their structure. The viscera are 

 entirely confined to the central disk ; and the arms are solid, like those of the Crinoidete, 

 being covered with a muscular integument, by which the joints are caused to move freely upon 

 I one another. On the other hand, they differ from the Crinoidea3 in having but a single aper- 

 ture to the digestive cavity ; and also in the position of the ovaries, which are here situated 

 xciOiin the disk, opening by separate orifices near the base of the arms. The arms are some- 

 times simple and undivided from their base to their free extremity, gradually tapering to a 

 point, as in the ordinary Ophiuru; ; whilst in Euryale they ramify minutely, dividing regu.. 

 Fig ] — [■"NeniNi's ^^^ly into branches, which again subdivide so as to form a most complex series of ajjpendages. 

 The arms in this order very commonly bear scales or spines on their surface ; and these 

 appear to be of great use to the animals, their roughness giving to the arms a point of rest, from which 

 they can push the body onward in any direction. Their movements are very active, in comparison with those 

 of Star-fishes ; and as they depend upon their spines for locomotion, they may be properly designated Spinj. 

 GUADA. Their ctrr7ti are not sufficiently developed to assist in locomotion ; although those near the mouth are 

 enlarged into tentacula, which seem to draw the food towards the orifice. 



III. In the AsTERiADiE, or true Star-fishes, the real arms altogether disappear ; the mz/s being merely lobes of 

 the body. In some instances there is scarcely any central disk, the body being aloiost entirely divided into rays ; 

 whilst in other cases there is but a slight division of the margin of the disk. The general structure of the 

 Asterias, which is the type of the order, is described in the text (p. 639). The movement of these animals is 

 sluggish, and is accomplished by means of the cirrld, \vhich form rows along the under side of the rays, and 

 which serve as suckers for taking an attachment to any solid body. Hence the order may be designated Ciebhi- 

 GBADA. The development of the Star-fishes has been recently studied by Sars and others ; and it appears that 

 they are attached in their embryonic condition, by a sort of footstalk divided at its base into three lobes. 

 This pedicle contracts, however, as the disk is developed, and is at last entirely withdrawn into the body of the 

 animal. A trace of it still remains, however, in what has been termed the madreporiform tubercle. 



IV. The order EcHiNiD^E corresponds with Cuvier's .S'ecDJid J i/mti/ V PedtrtWita [Tk^s^t p i 4(i) The type of the 

 order is the genus Echinus, 

 in wliich the shell is of globu- 

 lar form, having the oral 

 orifice at one pole, and the 

 a nus at the other. T he 

 mouth is furnished ^^"ith a 

 complex dental apparatus 

 (c. Fig. 2) ; and there is a 

 regular intestinal tube, 

 which makes two turns 

 ivithin the shell. The ova- 

 ries ojicn by distinct orifices 

 arountl the anus. The 

 movement of these animals 

 is partly accomplished by 

 their spines, which are fre- 

 quently very large and 

 strung ; and partly by their 

 cirrJd, which are always ca- 

 pable of being extended 

 further than the spines, and 

 of taking an attachment to 

 fixed bodies beyond. In this 

 manner the globular shell 

 may be drawn onwards in ' 

 any direction ; the movement being effected by the contraction of the tubes, but the body being supporte^I upoH 

 the .spines. From this compound mode of progression, the Echiiddcc may be termed Cimtni-SpiNiGHiBA, 1b 



T\c..1 — Anatomy r^F Echixc? 



!■ the teeth an 

 ovary;/,/, a 



; h. (Psophagiv ». 

 ■sides; g,g,»h-t^ 



