3260 ECHINODERMS 



and knotty, which communicate with the exterior at the angles between the rays. 

 These are the generative glands. In all Echinoderms, except sea cucumbers, these 

 glands are affected by the radiate structure of the animals; in crinoids the generative 

 products are even produced in the extremities of the arms. 



Having glanced at those points of structure in which Echinoderms 

 13 tfa cl" resem ^ e one an ther and differ from the rest of the animal kingdom, 

 we may shortly examine the main characteristics in which a sea 

 urchin, a starfish, a crinoid, a brittle star, and a sea cucumber differ from one 

 another. First may be noted obvious differences in form and in position in the liv- 

 ing state. In an ordinary sea cucumber (as shown in the illustration on p. 3258) 

 the body is cucumber shaped, with the mouth at one end and the vent at the other; 

 between these run the five ambulacra, one or two of which are often more developed 

 than the others, so that the animal crawls along on that side of its body, with its 

 mouth foremost. A sea cucumber has no arms or projecting rays, but its mouth is 

 surrounded by a circlet of tentacles, often branched, which can be retracted at will. 

 A regular sea urchin, such as the sea egg {Echinus}, shown in a later figure, resem- 

 bles a sea cucumber in being without projecting rays; but it is more spherical in 

 shape, and moves with its mouth toward the sea floor. On the other hand, in a 

 heart urchin {Spatangus} , which moves through and swallows mud and sand, the 

 body has become transversely elongate; that is to say, the long axis is at right 

 angles to the position it occupies in a sea cucumber; the mouth having moved a 

 little forward, and the vent being transferred from the top of the body to its lower 

 surface, so that both the mouth and vent lie on the under surface, at either end of 

 the long axis. In a starfish, as in a regular sea urchin, the mouth is in the centre 

 of the under surface, while the vent is almost in the centre of the upper surface, al- 

 though absent in a few forms. The body is either markedly pentagonal in outline, 

 or more or less star shaped. In -the latter case it is said to consist of a central disc 

 extended into arms, as in the illustration on p. 3271. The number of these arms 

 varies from five (Asterias) to over forty (Heliaster); but in each species with more 

 than six arms the number may vary slightly, although constant during the life of 

 the individual; in Labidiaster, however, fresh arms grow out even in the adult. A 

 brittle star (illustrated on p. .3258) resembles a starfish in which there is a sharp dis- 

 tinction between arms and disc; the mouth being on the under surface, but the vent 

 wanting. And whereas the arms of a starfish are simply extensions of the body, 

 containing the generative glands and processes from the stomach, those of a brittle 

 star are mere appendages to the body, with a stout internal skeleton of separate 

 ossicles, working on one another by well-developed muscles, and containing only 

 blood vessels, water vessels, and nerves. The arms of the brittle stars are nearly 

 always five in number, though sometimes there may be from six to eight. As in 

 the starfish, the arms are unbranched, except in the family Astropky tides, where 

 they fork ten or twelve times, and where the numerous branches interlace so as to 

 form a kind of basket work all round the disc, whence these animals are called 

 basket fish, or medusa head starfish. A crinoid (illustrated on p. 3264) differs 

 markedly from a sea urchin, starfish, or brittle star, in that the mouth faces upward; 

 the vent being also on the upper surface. This position is due to the fact that, so 



