THE ORDER AMONG ANIMAL TYPES 109 



Class I. ASTEROiDEA (Fig. 45). The Starfishes. 

 Class 11. oPHiuRoiDEA. The Brittle Stars. 

 Class III. ECHiNoiDEA (Fig. 47). The Sea Urchins. 

 Class IV. HOLOTHUROiDEA. The Sea Cucumbers. 

 Class V. cRiNoiDEA. The Feather Stars. 



Phylum Annelida 

 Members of this phylum (Figs. 48, 49, 50) represent a com- 

 paratively high state of organization when contrasted with the 

 forms already given here. The nervous system is well organized 

 and condensed into definite tracts and aggregations of cells termed 

 GANGLIA. The body is derived from three layers and the coelom is 

 well developed. The body contains a series of tubes which conduct 

 fluid to and from the tissues. This fluid is blood and contains a red 

 coloring material identical with that in the human blood. The gen- 

 eral structure is that of a tube within a tube, the digestive system 

 being the inner tube, ventral to which is the main nerve tract. From 

 anterior to posterior, the body consists of a series of rings, each much 

 like adjacent rings, termed metameres or segments. This type of 

 segmental arrangement is also characteristic of higher forms, in- 

 cluding the early stages of development in Man. Three classes of 

 Annelida are recognized. The most commonly known examples of 

 the phylum are the Earthworms and the Leeches. 



Phylum Moll us ca 

 The phylum Mollusca is composed of a very great number of 

 different forms, many of which, to outward appearances at least, 

 do not appear to be related. However, all have in common the 

 following features: Soft unsegmented bodies, usually protected by 

 a shell and derived from three layers; a ccelom is always present. 

 The symmetry is usually bilateral. The body wall is always pro- 

 tected by a MANTLE which forms a characteristic mantle fold; when 

 a shell is present it is formed by the mantle and the type of shell 

 depends on the type of mantle. A single muscular foot is always 

 present. There are five classes: 



