292 



ECHINODERMA 



urchins, trivium of holothurians). One pole of the axis of radial sym- 

 metry is marked by the mouth, which in the echinoids, starfish and brittle 

 stars, is turned downward; the other is frequently indicated by the anus. 

 The structure of the integument gives these animals a characteristic 

 appearance. Calcareous plates arise in the mesoderm, under the epithe- 

 lium, which form a body armor or test, and since these are usually produced 

 into spines, they give the name Echinoderma, spine skin, to the group. 

 This skeleton at times becomes degenerate, as in the Holothurians (ft 

 rarely entirely disappears as in Pelagothuria), but even then shows itself 

 as 'anchors' and 'wheels' of lime. The sp/iccridia and pedicellaria, com- 

 mon in echinoids and asteroids, are characteristic appendages of the in- 

 tegument. The first are sense organs; the latter are usually stalked 

 forceps-like grasping structures with calcareous skeleton. In life they 



are active and apparently either clean 

 the skin or are defensive. They are oc- 

 casionally provided with poison organs. 



Certain plates possess a morphological 

 interest since they appear early in many 

 larvae, and in the adults of different classes 

 can be recognized in similar positions. In 

 the neighborhood of the anus are five 

 basalia, interradial in position, farther five 

 radialia ('apical skeleton') and five inter- 

 radial 'oralia' around the mouth. 



Not less characteristic than the 

 skeleton is the ambulacral (or water- 

 vascular) system (fig. 281). This is a 

 system of ciliated tubes which begins 



FIG. 281. Water- vascular system 

 of starfish (orig.). a, ampullae; ab, 



\ O ' f IT i 



ambulacra; c, radial canal; OT, madre- usually at the surface, ordinarily by a 



porite; n, radial nerve; p, Polian vesi- , 



ele;r, ring canal, beneath it the nerve calcareous plate, the madreponte, per- 



nng; s 

 vesicle. 



stone canal; t, racemose f orated with fine pores for the entrance 



of sea water. The water passes into a 

 tube which, on account of its calcified walls in the starfish is called the 

 stone canal, and leads orally to a ring canal around the mouth. The 

 ring canal bears usually several (up to five pairs) of Polian vesicles, 

 which, with Tiedemann's vesicles of the starfishes, are now regarded as 

 appendages which, like lymph glands, produce leucocytes. From the 

 ring canal radiate five radial canals which give off, right and left in 

 pairs, the ambulacral canals. These in turn connect with the ambulacra 

 and ampullae, the highly characteristic locomotor organs of the echino- 

 derms. An ambulacrum is a muscular sac which can be distended and 

 lengthened by forcing in fluid from the ambulacral vessels, and is retracted 



