6 ECHINODERMA. 



A Sea-Urchin is provided with a most effective comminuting organ 

 in the shape of the pyramidal " Lantern of Aristotle," in which 

 work five strong teeth, ending in a wedge like the teeth of a rat 

 or other rodent. This Lantern is connected with a circlet of 

 upstanding pieces placed just inside the margin of the mouth 

 (actinostome of Agassiz), and consisting of the five " auricles " as 

 they are called. The dental apparatus may undergo reduction and, 

 finally, in the Heart-Urchins and Spatangus it disappears alto- 

 gether, and the creature is absolutely edentulous ; such Urchins 

 have the margin of the mouth produced into a spout, by means 

 of which they bore in the sand in which they live and from which 

 they extract what nutriment they can. 



The material which is undigested passes out by an anus in all 

 but a few Starfishes and all Brittle-stars, which are aproctous. 

 The anus is either exactly opposite the mouth and quite distinct, 

 as in Holothurians and the regular Sea-Urchins ; or it shifts towards 

 the margin and even on to the oral surface, as in the irregular Sea- 

 Urchins ; or it is on the back, but not quite central and generally 

 small, as in the proctuchous Starfish ; or it is on the oral surface, 

 near the mouth, and produced into an anal cone, as in Crinoids. 



Respiratory Organs. — The podia are both locomotor and respiratory 

 in function : as they project from the surface of the body, have com- 

 paratively delicate walls, and are, from time to time, filled with fluid, 

 they may be supposed to be always respiratory in a general way ; 

 but sometimes they are so flattened or pointed that they clearly can 

 have no sucking action, and are purely respiratory. Special organs 

 for respiratory purposes are not always developed, or, if so, are of 

 a purely generalized character, as the soft membranous pouches or 

 saccules which project from among the interstices of the calcareous 

 skeleton of a Starfish. In an ordinary Sea-Urchin there may be 

 seen on the membrane around the mouth (buccal membrane) five 

 pairs of small folded processes, lodged in narrow slits of the margin 

 of the test; these are known as gills and the slits as the gill-slits. 

 In the Piper ( Cidaris) these gills are wanting, but there are about 

 the Lantern of Aristotle five large membranous sacs which are sup- 

 posed to have a respiratory function ; they are known as the Organs 

 of Stewart, after the name of their discoverer. In the Ophiuroids 

 there is, on each side of the base of every arm, a slit which leads 

 into a pouch or sac ; into and out of this sac water passes, and as 

 its walls are thin an exchange of gas is effected between the fluid 

 in the body-cavity and the water which is pumped in and out ; 

 these sacs are known as bursal sacs, or bursas. In Holothurians 

 water may be, as in some species of Stichopus, pumped into and 

 out of the intestine, or from the terminal portion of the digestive 

 tract a trunk on either side may give rise to a more or less 

 elaborately branched " respiratory tree " or system of fine and 

 delicate tubes by means of which water carrying fresh oxygen 

 comes into all but contact with the fluid in the body- cavity; lastly, 

 some Holothurians have no special respiratory organs, and have 

 consequently been spoken of as Apneumona. 



