138 LOWER INVERTEBRATES. 



the stone canal. Regarding the functions of this system opinions differ greatly. 

 Some consider it as a true circulatory system, the tube connecting the two rings being 

 regarded as a heart, and described as pulsating in life. Perrier, the French authority 

 on these forms, on the other hand, regards the so-called heart of the star-fishes, brittle- 

 stars, and sea-urchins, and the corresponding dorsal organ of the crinoids, as glandular 

 and the connecting branches as coeca. 



The nervous system consists of a ring around the mouth from which radial branches 

 follow the course of each ambulaoral area. Subdivisions of these principal nerves 

 supj>ly the ampullae, ambulacra and other parts of the body. The only sense which 

 appears to be present in all of the group is that of touch, for which various external 

 parts are well adapted. In some of the star fishes and sea urchins rudimentary eyes are. 

 found at the extremities of the ambulacral areas, whether at the tip of the arms or at 

 the corresponding position on the aboral surface. (Fig. 124, c.) 



The external covering varies greatly in the various forms, but plates of carbonate 

 of lime are usually present. These plates may be firmly united so as to form a solid 

 shell, or they may be separate and imbedded in the integument. In some they may 

 take the form of spicules, wheels, and anchors, those of Chirodota and Synapta being 

 familiar objects to all workers with the microscope. In the star fishes these plates are 

 very numerous, two series roofing over the ambulacral groove in which the sucking 

 feet are situated. In the serpent stars the ambulacral plates occupy the interior of the 

 arm, which is entirely surrounded by a series of plates. In most of the sea urchins 

 the jjlates of the ambulacral and inter-ambulacral areas unite to form the solid test in 

 which the body is usually enveloped, the surface of which is covered with little 

 rounded prominences for the attachment of spines. In the crinoids the calyx and the 

 jointed stalk and arms are largely composed of calcareous matter. 



Spines, often varying greatly in size in different portions of the same individual, 

 are widely distributed among the Echinodermata, most of which also possess certain 

 fork-like or pincer-like organs, which are modified spines, and which from being 

 stalked in some forms are known as pedicellariiE. These are capable of closing and 

 seizing any object which may come between their jaws, and are supposed at least in 

 some forms to take the excrement from the anus on the upper surface of the body, 

 and pass it along, off from the shell to the ground. 



The sexes of most echinoderms are separate, and the genital products are discharged 

 either by a breaking away of the integument or by true genital openings. The 

 young of most of the branch undergo a wonderful metamorphosis in the course of 

 their development, and the embryos are free swimming animals. The ' pluteus ' of 

 the sea urchin, the ' bipinnaria ' or the ' brachiolaria ' of the starfish, and the ' auricu- 

 laria ' of the holothurian, bear no resemblance to the adults, and in fact the name now 

 applied to these larvae were originally given them under the impression that they were 

 adults. In certain groups the embryo develops into -the adult without any metamor- 

 phosis. As there is such variation among the different groups we will defer the 

 details of development until treating of the respective forms. 



The Echinodermata are all marine, and are found in all the seas of the globe. If 

 ancestry confers respectability these forms should be classed among the nobility, for 

 remains of these animals are found in some of the oldest fossiliferous rocks. At the 

 present time they play an unimportant part in the economy of the world, and are of 

 but slight importance to mankind. A few forms are used as food, while others are 

 injurious to human interests, as they destroy beds of oysters and other shell-fish. 



