CHIEF DIVISIONS OF THE ANIMAL KINGDOM. 83 



with the exterior, and which constitute what is called the " water- vascular 

 system." The body of the adult is never composed of a succession of 

 definite rings, or provided with successive pairs of appendages disposed 

 symmetrically on the two sides of the body. 



The Annuloida are divided into two great classes : 



A. ECHINODERMATA. Integument composed of numerous calcareous 

 plates joined together, or leathery and having grains, spines, or tubercles 

 of calcareous matter developed in it. Water- vascular system (ambulacral 

 system) mostly employed in locomotion, and generally communicating 

 with the exterior. Adult generally more or less star-like or " radiate " 

 in shape ; young mostly showing more or less complete " bilateral sym- 

 metry," that is, showing similar parts on the two sides of the body. 

 Nervous system radiate. 



Order 1. Crinoidea (Sea-lilies). Ex. Feather-star (Comatula), Me- 

 dusa-head Crinoid (Pentacrinus), Stone-lily (En- 

 crinus). 



Order 2. Blastoidea (extinct). 



Order 3. Cystoidea (extinct). 



Order 4. Ophiuroidea (Brittle-stars). Ex. Sand-stars (Ophiura), 

 Brittle-stars (Ophiocoma). 



Order 5. Asteroidea (Star-fishes). Ex. Cross-fish (Uraster), Sun- 

 star (Solaster), Cushion-star (Goniaster). 



Order 6. Echinoidea (Sea-urchins). Ex. Sea -eggs (Echinus), 

 Heart-urchins (Spatangus). 



Order 7. Holothuroidea (Sea-cucumbers). Ex. Trepangs (Holo- 

 thuria). 



B. SCOLECIDA. Body usually flattened, or cylindrical and worm-like ; 

 integument soft, without lime. Water-vascular system not assisting in 

 locomotion. Nervous system consisting of one or two ganglia or little 

 masses, and not disposed in a radiate manner. 



Order 1. Tceniada. Ex. Tape-worm (Tsenia). 



Order2. Trematoda (Suctorial worms). Ex. Liver-fluke (Dis- 



toma). 

 Order 3. Turbellaria. Ex. Planarians (Planaria), Ribbon-worms 



(Nemertes). 

 Order 4. Acanthocephala (Thorn -headed worms). Ex. Echino- 



rhynchus. 



Order 5. Gordiacea (Hair-worms). Ex. Gordius. 

 Order 6. Nematoda (Thread-worms). Ex. Round-worm (Ascaris), 



Guinea-worm (Filaria), Vinegar-eel (Anguillula). 

 Order 7. Rotifera (Wheel-animalcules). Ex. Builder -animalcule 



(Melicerta), Flexible Creeper (Notommata). 



SUB-KINGDOM I V.A NNULOSA . 



Animal composed of numerous definite segments or " somites;" arranged 

 longitudinally, one behind the other. Nervous system always present, 

 consisting typically of a double chain of nervous masses, or ganglia, 



