Echinoderma. 123 



(to be described in detail below), and alternate regularlv with the 

 other five, called inter-radii. 



The radial tvpe of structure is not onlv indicated extemaU}-, bat is 

 also conspicuous in most of the internal organs \Kee below), although 

 it is never completely carried out ; there are always deviations, in 

 some systems at least, and these are very considerable in many 

 forms (for details, see the various groups, especially the Echinoids). 



It is characteristic of the Echinodenns that, in almost all, 

 calcifications of varving size and form occur in the connective 

 tissue of the body-wall. Sometimes thev are small, almost micro- 

 scopic deposits, often very beautiful in form : small perforate plates, 

 wheels or anchors ; sometimes larger landnae movably or immovably 

 ci_iunected together. With the exception of some quite small deposits, 

 aU the calcifications are porous and spongy. In most cases they are 

 present in such numbers that they form a considerable portion of the 

 whole ma«s ; occasionally (Holothnrians) they are more subordinate.* 

 Calcifications may occur not only in the body-wall, but also iu other 

 regions, e.g., in the wall of the stone-canal {see below) and in the 

 pharynx of the Holothurians. 



The skin is, as a rule, ciliated and often brightly coloured. In 

 connection with it, there are various appendages, many of which 

 are calcified like the body-wall. This is the case for instance in the 

 movable s p i n e s"*" usually present, part of which is calcareous 

 matter, although connective tissite and epidermis are by no means 

 wanting. A peculiar form of appendage, the so-called pedicellariae, 

 also occurs in Starfish and Sea-urchins. Each consists of two or 

 three short calcified valves connected at one end. whilst the free ends 

 are often provided with curved tips capable of snapping together like 

 pincers ; the pedicellarite are usually borne upon a longer or shorter 

 movable stalk, supported proximally by a calcareous ri:>d. They serve 

 as defensive organs; small animals are seized and held fast until 

 they die ; excreta and foreign particles are also removed from the 

 surface of the animal by their agency. 



Among the appendages of the Eehinodenus, the tube-feet are 

 of special interest ; they are soft, delicate, usually cylindrical 

 structures, the free ends of which are either furnished with suckers 

 or rounded; only in the former case do they serve as organs of 

 attachment. They may stretch out to a considerable length, and 

 then have the appearance of very long thin threads, whilst in the 

 contracted condition, they shrink up to a small fraction of their former 



* The calcifications lie in connective tissne covered by the epidermis. In regions 

 where the body is exposed to friction, the soft covering may, however, be rubbed off, 

 so that they become partially bare (tips of the spines of Sea-nrchins, portions 

 of the surface of the Ophinrids, etc). 



+ Frequently the spines are not altogether simple in stmctnre, but are forked and 

 so on. Such e g. are the paxillse of some Starfish, bearing; a rosette of fine points 

 at the end of the shaft. 



