306 Animal Biology 



bristles, are common in many species. The animals are triploblastic, having three 

 primary germ layers: ectoderm, mesoderm, and entoderm. All species possess a 

 true coelom (body cavity), although it may be small in some types, such as 

 leeches. They possess a complex closed circulatory system. Some species are 

 aquatic, some terrestrial, and others marine. Number of species of Annelida, 

 7,500. 



Calcareous 



Ring canal 



Madrepor'ite cana] . . 

 and plate .^ 



tsophaqus 

 (^nita) duct 



Stomach 

 (Jonad... 



Oral tcntades 



Retractor muscle 



Polian vesicle 



Respiratory' 

 tree 



'-Mesenteries 

 '-- Intestine 



v.. ..Longitudinal 

 muscles 



Cloaca muscle 



Cloaca.... 



Lloaca aperture 



Fig. 111. — Sea cucumber (Thyone briareus) of the phylum Echinodermata, 

 class Holothurioidea, shown in longitudinal section and somewhat diagrammatically. 

 (From Coe: Echinoderms of Connecticut, State Geological and Natural History 

 Survey of Connecticut, Bulletin 19.) 



Classification of the Phylum Annelida 



Class 1 — Archiannelida (ar ki a -nel' i da) (Or. archi, primitive; annelida). 

 — These are primitive marine annelids which lack both bristlelike setae and para- 

 podia (flat organs of locomotion and respiration). 



