COMPARATWE ANATOMY 



infusoria that swarm in water containing organic matter. Some are 

 parasitic, and produce sleeping sickness, sj'philis, and other diseases. 

 Some contain chlorophyl and might about as reasonably be counted as 

 plants. A few colonial forms, for example, the common green volvox, 

 have their individuals differentiated into vegetative and reproductive 

 cells, and thus take the first step toward the evolution of multicellular 

 organisms . 



The protozoa, which are single-celled and, apparently, simple, make a 

 logical starting point for animal evolution. The amoeba, which is among 

 the simplest of the protozoa, is popularly assumed to be "the mother of 

 all living." This assumption is in no ^^nse unreasonable as there is no 

 direct evidence to the contrarv. 



BlASTDPOR£CM<Xn>h»MJS> 

 A COeLENTIRATE. 



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B.FLATWORM. 



aLAST::PCfl£CVOJTH) 

 2 MOLLUSC. 



BCA5T0POSE CanUSJ 



BLASTOPORE (ANUS 



BlasTO<>0«C 



C»CUR£NTCR1C 



STOPOBE 



€. EOHINOOCRM. 



MOJTH 

 F. «MICH0ROATE. 



CUKXHOPDATZ. 



KCEPHAaXhORQME. 



Fig. 3. — Diagrams of embr\-onic stages illustrating the contrast in the fate of the 

 blastopore in different groups of animals. The forms in which the embryonic blastopore 

 becomes the mouth were grouped together by Grobben as PROTEROSTOMIA. The 

 DEUTEROSTOMIA include those animals in which the blastopore becomes the anus 

 or lies in the anal region. The coelenterates, flatworms, annelids, and molluscs are 

 Proterostomians, while echinoderms and chordates are Deuterostomians. 



Multicellular animals or Metazoa have bodies which consist of many 

 cells. Grobben, 1908, di\'ides the metazoa into two main subdivisions: 



I. Proterostomia, in which the embryonic mouth or blastopore persists 

 as the adult mouth. Proterostomians include the Platyhelminthes, 

 Nemathelminthes, Rotifera, Molluscoida, Annelida, Mollusca, and 

 Arthropoda. 



II. Deuterostomia, in which the blastopore forms the anus or Ues near 

 the anus. The deuterostomians include the remaining groups of animals, 

 — Echinodermata, Hemichordata, Urochordata, Cephalochordata and 

 Vertebrata. 



Phylum 2. PORIFERA 



The differentiation among individual cells, which began faintly in 

 certain colonizing protozoa, is carried somewhat further in the sponges, 

 which, though rated as multicellular animals, are hardly more than sHghtly 



