GENERAL CHARACTERS. 293 



what branched 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. 

 Distinction of Having glanced at those points of structure in which Echinoderms 



the classes, resemble one another and differ from the rest of the animal kingdom, 

 we may shortly examine the main characters in which a sea-urchin, a star-fish, 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 living state. In an ordinary 

 sea-cucumber (as shown in the illustration on p. 291) 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, resembles 

 a sea-cucumber in being without projecting rays ; but it is more spherical in shape, 

 and moves with its mouth towards 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 star- fish, 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, although 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. 304. 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. 291) resembles a star-fish 

 in which there is a sharp distinction between arms and disc ; the mouth being 

 on the under surface, but the vent wanting. And whereas the arms of a star-fish 

 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 star-fish, the arms are unbranched, 

 except in the family Astrophytidce, 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 star- 

 fish. A crinoid (illustrated on p. 297) differs markedly from a sea-urchin, star-fish, 

 or brittle-star, in that the mouth faces upwards ; the vent being also on the upper 

 surface. This position is due to the fact that, so far as we know, all crinoids are 



