6 



and two (usually) approximated ventral cords. Closed vascular system. 

 Excretory organs — paired tubes in each segment, opening externally and 

 into body cavity. 



1. PoLTCHiETA (mauy-bristled). — Marine forms. Outgrowths of body 



wall form limbs (parapodia), on which are placed numerous horny 

 bristles (setce). Outgrowths of body walls form respiratory organs 

 (gills), e.g. Arenicola (log-worm), Aphrodite (sea-mouse). 



2. Oligoch.«:ta. (few-bristled). — No gills. No parapodia. Never more 



than eight setffi in each bundle. Hermaphrodite, e.g. Lumbricus 

 (earth-worm). 



3. AcH^ETA. — No setsB. No parapodia. e.g. Polygordius. 



VI. HIETJDINEA (Discophora, Leeches). — Elongated. Segmented. 

 Sacker at one or both ends. No setas or parapodia. Gut is pouched. 

 Body-cavity filled up with spongy tissue. Excretory and nervous systems 

 as in Cheetopods. Hermaphrodite, e.g. Hirudo (Leech). 



Class v.— ECHINODERMATA (Spiny-skinned 



Animals). 



Characters. — Echinoderms are exclusively marine Metazoa — (1) in which 

 a bilateral symmetry (apparent in the young) is obscured by an acquired 

 radial (usually pentamerous) symmetry ; (2) sJtin is hardened by limy 

 deposits, which may be so abundant as to form a continuous skeleton; 

 (3) a water-vascular system with "tube feet" is present; (4) neroous system 

 in the form of u ring round the gullet and branches to the " arms " (rays). 



SXJB-DIVISIONS. 



L CRINOIDEA (Lily Htars).— (1) Echinoderms, with a skeleton 

 (calyx), which is a cup -like structure mostly prolonged at one end into 

 a jointed stem, by which the animal is mostly attached, and at the other 

 (the mouth and anus end) forms (usually) branching arms, into which the 

 water-vascular, vascular, and nervous systems extend. e.g. Rosy-feather 

 Star (which is stalked only in the larval form). 



II. ASTEROIDEA (Star-fishes).— (1) Flattened bodies, either star- 

 shaped or a pentagonal disc ; (2) special ambulacral ossicles or plates 

 protect and support the tube-feet, and form the ambulacral groove, along 

 which run branches of the vascular, water-vascular, and nervous systems ; 

 (3) tube-feet confined to under surface ; (4) mouth ventral. Anus dorsal. 

 e.g. Asterias (star-fish), 



III. OPHIUROIDEA (Brittle Stars).— (1) Star-shaped Echinoderms, 

 formed of a central disc and moveable, often branched, arms; (2) ambulacral 

 groove closed in below; (3) tube-feet confined to under surface; (4) anus 

 absent; (5) more concentrated than star-fishes since alimentary canal is 

 confined to the central disc, and does not, as in star-fishes, extend into the 

 arms. 



