202 



ECHIXODER.MA 



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

 metry is marked Ijv the mouth, wliieli in ilie eehinoids, starlish and lirittle 

 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 bodv armor or test, and since these are usually produced 

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

 Tliis skeleton at times becomes degenerate, as in the Holothurians (it 

 rarelv entirelv disappears as in rdagothuria), but even then shows itself 

 as 'anchors' and 'wheels' of lime. The splucriJia and pcJiccllaria, com- 

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

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

 forcepsdike grasping structures with calcareous skeleton. In life they 



are acti\-e 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 manv 

 larva\ and in the adults of dilTerent classes 

 can be recognized in similar positions. In 

 the neighborhood of the anus are five 

 casalia, interradial in position, fanlicr li\e 

 rad:alia ('apical skeleton') and li\-e inter- 

 radial 'oralia' around the mouth. 



-■Tot less characteristic tlian the 

 skeleton is the ambulacra! (or water- 

 vascular) syslcni (hg. 2S1). Tliis is a 

 system of ciliated tubes wliich begins 

 usually at the surface, ordinarilv bv a 

 calcareous plate, tlie maJn-pori/r, per- 

 forated with hne pores for the entrance 

 of sea water. The water passes into a 

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

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

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

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

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

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

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

 and ampulkt;, the highly characteristic locomotor organs of the ecliino- 

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

 lengthened by forcing in duid from the amhulacral vessels, and is retracted 



Fig. ::Si. — A\'alcr-vascular system 

 of slarllsli (orig.). a, ampulla'; ah. 

 ambulacra; c, radial canal; m, madre- 

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

 cle: /', ring canal, beneath it the nerve 

 ring; s, stone canal; t, racemose 

 vesicle. 



