392 



Comparative Morphology of Chordates 



the earliest metazoans must have been somehow derived from ancient 

 protozoan ancestors. 



The dichotomous branching of the "tree" in Fig. 301 illustrates 

 a recent view for which there is some embryologic evidence. Accord- 

 ing to the "gastraea" theory of Haeckel (see p. 349), the two-layered 

 gastrula stage of the embryo, which commonly occurs throughout 

 Metazoa, indicates common ancestry of all metazoans from two- 

 layered coelenterates (Fig. 285). The digestive cavity of the coelenter- 

 ate has only one external opening, the mouth or "mouth-anus." 

 The gastrular cavity, which is the prospective digestive cavity of the 

 adult, opens to the exterior by the "embryonic mouth" or blastopore. 

 It is a striking fact that, in some metazoans (Fig. 302C, D), the blas- 

 topore persists as the adult mouth, or at least marks the oral end of the 

 animal and, becoming closed, is replaced by the definitive mouth, 

 whereas, in other metazoans (Fig. 302E-H), the blastopore either 

 persists as the adult anus or marks the prospective anal region of the 

 embryo, the mouth then developing at the opposite end as an entirely 

 new opening. This completely opposite polarity of embryos, in some 

 the blastopore marking the head end, in others the tail end, seems a 

 matter of quite fundamental importance. It may justify showing the 

 metazoan "tree" as splitting, just above the level of the sponges, 

 coelenterates, and flatworms, to form two grand trunks. One of them, 

 the Protostomians (or Proterostomians; on the left in Fig. 301), 

 includes the phyla in which the adult mouth is formed at the blas- 

 toporal region of the embryo or may even be the persisting primary 



E. ECHINODERM. F. HEMICHORDATE G. UROCHORDATE H CEPHALOO-ORDATE 



Fig. 302. Diagrams of embryonic stages illustrating the contrast in the fate of 

 the blastopore in various 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 mollusks are 

 Proterostomians, while echinoderms and chordates are Deuterostomians. (Cour- 

 tesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston 

 ( !ompany.) 



