2 ECHINODERMS OF THE BRITISH ISLES 



developed, consisting of scattered microscopical plates which may 

 be of a very elegant shape. The skin then generally gives the 

 impression of being quite naked. Even in those sea-cucumbers 

 (Psolus) which have a more developed skeleton, the plates never 

 show such regular arrangement as in other echinoderms. The 

 skeleton is formed inside the epidermis and is covered with skin, 

 which is mostly finely ciliate. 



To the skeleton various structures may be attached, viz. 

 spines, pedicellarise, and sphaeridise. The spines may reach a 

 considerable size, and are usually articulated to special knobs 

 and provided with muscles, being thus movable in all directions ; 

 they are found in sea-urchins, sea-stars, and brittle- stars. The 

 pedicellaricE are peculiar organs, often of very complicate struc- 

 ture, sometimes provided with poison glands. Their function 

 is partly to clean the skin of dirt and all kinds of foreign bodies, 

 partly to catch and hold a pre}^ and partly to serve as defence 

 organs. They are found only in sea-urchins and sea-stars. The 

 sphceridia, which are supposed to be organs of taste and smell, or 

 of orientation, are found only in sea-urchins. 



The tube-feet are cylindrical, highly extensible tubes, generally 

 ending in a sucking disk. Primarily they occur in two series in 

 each radius, but their number may be augmented so as to form 

 four series, or so as to be arranged in arches, or they may be wholly 

 without definite order. In some sea-urchins, and especially sea- 

 cucumbers, they even extend over the interradii, the whole body 

 thus being covered with tube-feet. In sea-stars and sea-lilies they 

 are placed in furrows (ambulacral furrows or grooves) along the 

 arms. The tube-feet mainly serve as locomotor organs. When 

 distended the sucking disk is brought into contact with the 

 substratum ; muscles, which are fastened to the middle of the 

 disk, contract, and thus produce a small, empty space. The air 

 or water pressure then attaches the sucking disk so tightly to 

 the substratum that the animal can move itself forwards by 

 shortening the tube-foot. So tightly do they adhere that on catch- 

 ing a specimen thus attached numbers of tube-feet are pulled off, 

 the sucking disks torn off remaining attached to the substratum. 

 The tube-feet are connected with the watervascular system ; 

 this system is peculiar to the echinoderms, nothing quite corre- 

 sponding to it being found in any other group of animals. 



The watervascidar system (Fig. 1) consists of a ring canal, 

 encircling the oesophagus, and five radial canals issuing from it, one 

 for each radius : in the many-rayed forms there is a corresponding 

 number of radial canals. From the radial canals pairs of small 



