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Mention the Chief Classes of Echinoderms and give Ex- 

 amples. 



"Class ASTEBOIDEA (Starfishes). The body, flattened dorso-ventrally, 

 is stellate or pentagonal, and has five or more distinct rays or " arms." 

 Each " arm " contains diverticula of the gut and gonads, and has 

 a ventral ambulacral groove bearing two or four rows of locomotor 

 tube -feet. 



Examples, Asterias (The Common Starfish). 



Solaster (Sun -star), with nine or more rays. 

 Goniaster (Cushion -star). A pentagonal form. 

 Class OPHIUROIDEA (Brittle-Stars). 



Class ECHINOIDEA (Sea-urchins). Spherical or heart-shaped. The 

 rays fused together. The exoskeleton a complete test or shell 

 composed of limy plates and set with movable spines. Ambulacral 

 grooves closed. Five double rows of slender locomotor tube-feet. 



Example, Echinus (Sea-urchin). 



Class HOLOTHUROIDEA (Sea-cucumbers). Body elongated and cylin- 

 drical. An exoskeleton of loose spicules. The body-wall muscular. 

 Mouth with contractile tentacles (modified buccal tube-feet) for 

 feeding. Locomotor tube-feet present or absent. Breathing by 

 " gill-trees," branched outgrowths of terminal portion of gut. 



Examples, Cucumaria. With non-retractile tube-feet. 

 Synapta. Without tube-feet, and with an 



exoskeleton of anchor-like spicules. 

 Class CKINOIDEA (Feather-stars). 



Give a General Account of the Water- Vascular System of 

 Echinoderms. 



On the outside of the body there is a small porous plate, 

 the madreporite, through which water is drawn into the 

 stone canal, which is lined with cilia and which leads vertically 

 into the circumoral or ring canal. The ring canal often has, 

 interradially, accessory reservoirs, the polian vesicles, and 

 small glandular pouches, Tiedemann's bodies, in which the 

 amoebocytes are formed. The polian vesicles, having 

 muscular walls, serve also to vary the pressure of the 

 fluid in the whole system. Radial canals, one in each 

 radius or ray, are given off from the ring canal ; arid the 

 radial canals give off small lateral branches, one to each 

 tube-foot. There is an ampulla or bladder-like reservoir 

 at the base of the tube-foot where the lateral canal enters 

 it ; and the lateral canal has a pair of valves which, under 



