BRANCH ECHINODERMATA : STARFISHES, ETC. 117 



body are usually five. In the body of an Echinoderm 

 can be usually recognized an upper or dorsal surface and 

 a lower or ventral surface. The mouth is usually situated 

 on the ventral side and the anal opening on the dorsal. 

 Echinoderms agree also in having a calcareous outer 

 skeleton or body-wall usually in the condition of definitely- 

 shaped plates or spicules fitted either movably or rigidly 

 together. This outer body-wall or exoskeleton may bear 

 many tubercles or spines. These spines are sometimes 

 movable. The body-wall of the sea-urchin shows very 

 well the exoskeleton composed of plates on which are 

 borne movable strong spines. 



Structure and organs. As has been learned from the 

 dissection of the starfish, the Echinoderms have well- 

 developed systems of organs. The body-structure in its 

 complex organization presents a marked advance beyond 

 the structural condition of the polyps and jellyfishes. 

 There is a well-organized digestive system with mouth, 

 alimentary canal, and anal opening. The alimentary 

 canal is either a simple spiral or coiled tube, or it is a tube 

 in which can be recognized different parts, namely, 

 oesophagus, stomach, intestine, caeca, and special glands 

 secreting digestive fluids. This alimentary canal is not, 

 as in the polyps, simply the body-cavity, but it is an in- 

 closed tubular cavity lying within the general body-cavity. 

 At the mouth-opening there is in some Echinoderms, 

 notably the sea-urchins, a strong masticating apparatus 

 consisting of five pointed teeth which are arranged in a 

 circle about the opening. ["The nervous system consists 

 of a central ring around the oesophagus or mouth, from 

 which branches extend into the radiately arranged arms 

 or regions of the body. There is no brain as in the 

 higher animals, but the central nerve-ring is composed of 

 both nerve-cells and nerve-fibres as in the nerve-centres 

 of higher forms. Of organs of special sense there are 



