THE EVIDENCE FOR EVOLUTION 



731 



is an extremely aberrant barnacle which parasitizes crabs. The adult 

 form is a saclike structure which sends processes into the tissues of the 

 host to absorb nourishment. It resembles no other organism and its rela- 

 tionship became clear only when it was found that its larva is like that 

 of other barnacles until it becomes attached to the abdomen of the host. 

 Then it loses its appendages and other structures and becomes the adult, 

 saclike creature. 



The concept of recapitulation is very helpful in understanding the 

 curious and complex development of the vertebrate circulatory and 

 excretory systems. It is also useful, when not taken too literally, in get- 

 ting a broad picture of the whole of development. Thus the fertilized 

 egg can be compared to the putative single-celled flagellate ancestor of 

 all animals, and the blastula can be compared to a colonial protozoan 

 or to some hypothetical blastula-like animal which has been postulated 

 to be the ancestor of all Metazoa. Haeckel believed that the ancestor of 

 coelenterates and all the higher animals was a gastrula-like organism 

 with two layers of cells and a central cavity connected by a blastopore 

 to the outside. After gastrulation, development follows one of two main 

 lines. In the echinoderms and chordates the blastopore becomes the anus, 

 or comes to lie near the anus. In the annelid-mollusc-arthropod line the 

 blastopore becomes the mouth or comes to lie near the mouth. In both 

 lines the mesoderm develops between the ectoderm and endoderm. In 

 the chordate-echinoderm line the mesoderm develops, at least in part, as 

 pouches from the primitive digestive tract, whereas in the annelid-mol- 

 lusc line the mesoderm usually originates from special cells differentiated 

 early in development. 



All chordate embryos develop, shortly after the mesoderm begins to 

 appear, a dorsal hollow nerve cord, a notochord and pharyngeal pouches. 



AorHc 



-"Ventricle.- 

 Atrium 



Ca.rdinal 

 veins 



Fiqure 35 6 Ventral views of the heart and aortic arches of a hurnan embryo 

 /ri^htf and an adult shark (left). Both have a single atrium and single ventricle, 

 sefral aortic arche!! Tnd anterior and posterior cardinal veins emptying into the 

 heart. 



