654 ECHINODERMA. 



as a system of branched tubes which convey blood through the body ; and in 

 these respects they differ from the jelly-fish, with which their radiate 

 symmetry has often caused them to be allied. 



A system of tubes conveying water from the exterior throughout the 

 body, and serving as a hydraulic system, is characteristic of the echinoderms. 

 On the under side of the rays in a living star-fish may be seen a number of 

 small cylindrical processes which wave about gently like trees in a wind 

 (Fig. 1, P). They lie in each ray along two rows with a clear space 

 between, as trees on either side of an avenue; hence the whole band of them 

 in each ray is called an ambulacrum (garden-walk). In the sea-urchin 

 similar rows of the tube-like processes are seen passing from the 

 summit down to the base, dividing the test into five equal portions. 

 In the star-fish and sea-urchin these processes end in sucker-like discs. 

 The tubular process can be extended to a considerable length and 

 its disc attached to some neighbouring object. The contraction of the 

 process then draws the animal along ; hence in these animals the processes 

 are called tube-feet. Similar processes occurring in the brittle-stars and 

 crinoids have no suckers at the end, and serve only for purposes of respira- 

 tion and not for locomotion. The processes in general may therefore be 

 called podia. Their movements are caused by the squeezing of water into 

 them from the internal water-vessels ; for each podium is like an indiarubber 

 tube, closed at one end, passing through the test to join with one main tube 

 which runs along under the ambulacrum in a radial direction (Fig. 3, W), 

 and before it joins this radial canal, each podium gives off a small swelling 

 likewise filled with fluid, so that when this swelling, or ampulla (Fig. 3, Amp.), 

 is contracted, all the fluid is squeezed up into the podium, and pushes it out 

 like the finger of a glove when one blows into it. All the radial canals 

 meet in a ring-canal surrounding the mouth, and this is connected with the 

 outside water by a canal that passes right across, the body-cavity to the 

 other side of the animal, where it opens to the exterior through a sieve-like 

 plate called the madreporite (Fig. 1, M). The walls of this canal are often 

 hardened by limy deposits, whence it is known as the stone-canal. Some 

 such system of water-canals occurs in all echinoderms, but in the sea- 

 cucumbers the madreporite that should connect it with the exterior is sunk 

 inward, and hangs in the fluid of the general body-cavity. Direct com- 

 munication with the exterior is perhaps not so necessary with these animals 

 since their flexible muscular walls enable them to drive the fluid of the body- 

 cavity in any desired direction. 



There are three main systems of nerves found in echinoderms, one supply- 

 ing the skin, the podia, and the gut, and consisting of a nerve-ring round 

 the mouth, with radial nerves passing from it beneath the ambulacra ; the 

 second, with a similar arrangement, but placed deeper, and supplying the 

 internal muscles of the body wall ; the third, which is best seen in crinoids, 

 starts from the other side of the body, opposite to the mouth, and supplies 

 the muscles that work the arms and stems. 



In the star-fish the generative glands are orange- coloured tubes somewhat 

 branched and knotty, which pass down the sides of the rays and communi- 

 cate with the exterior at the angles between the rays (Fig. 2, G) ; they have 

 a similar position in the sea-urchins; in crinoids they extend right down the 

 arms, at the extremities of which the generative products are produced, In 

 the sea-cucumber, however, there is but a single much-branched generative 

 gland. 



