RADIATA 



693 



Fig. 1 — ENCRiNi's. 



The Pentacrinus, however, must not bo regardeo as the true i,.>pe of the Crinoid order, but rather as a link of 

 transition whicli connects it with the higher forms of Echinodermata. For the bulk of the group is made up of 

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

 organization, connecting the free Echinodermata 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 those 

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

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

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

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

 two following. 



II. Ophiukid^. These Star-fishes are so named from the long serpent- or worm-like arms, 

 which are appended to their round, depressed, urchin-like bodies. Although commonly as- 

 sociated with 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 Crinoideae, 

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

 one another. On the other hand, they diifer from the Crinoidece in having but a single aper- 

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

 within 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 Ophiurce ; whilst in Euryale they ramify minutely, dividing regu- 

 larly into branches, which again subdivide so as to form a most complex series of appendages. 

 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 Spini- 

 ORADA. Their eirrhi 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 AsTEBiAD*, or true Star-fishes, the real ornis altogether disappear ; the rai/s being merely lobes of 

 the body. In some instances there is scarcely any central disk, the body being almost 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 

 Astenas, 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 eirrhi, which 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 Cibrhi- 

 OBADA. 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 Second Family of FediceUata (Text, p. 640). The type of the 

 order is the genus Echinus, 

 in which the shell is of globu- 

 lar form, having the oral 

 orifice at one pole, and the 

 anus at the other. The 

 mouth is furnished with a 

 complex dental apparatus 

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

 regular intestinal tube, 

 which makes two turns 

 within the shell. The ova- 

 ries open by distinct orifices 

 around the anus. The 

 movement of these animals 

 is partly accomplished by 

 their spines, which are fre- 

 quently very large and 

 strong ; and partly by their 

 eirrhi, 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 



nouth, stirroxinded by the trcth nnd Ja 

 !, intestinal tubcj r, ovury ; /, /, i 



rftl vesicles j f^,g, iihell. 



may be drawn onwards in '""""''h. or first portion of the i 



any direction ; the movement being efi'ected by the contraction of the tubes, but the body being supported! upon 



the spines. From this compound mode of progression, the Eehinida; may be termed f'liiRHi-SriNlGKADA. In 



