294 GENERAL ZOOLOGY 



animals that in the adult state resemble molluscs rather than 

 chordates. A sessile tunicate, such as Ciona, is tubular in form 

 and is provided with an incurrent and excurrent siphon through 

 which water passes in and out as in a clam. There is no trace of 

 a notochord and the central nervous system is represented by a 

 ganglion lying between the two siphons (Fig. 264). About the 

 only definite chordate character recognizable in the adult animal 

 is the presence in the pharynx of gill clefts, but even these are 

 highly specialized when compared with the gill clefts of other 

 chordates. The question as to the position of the group Tunicata 

 in the classification is settled by the presence in the motile, free- 

 swimming, tailed larval stage of a well-developed dorsal 

 neural tube and of a notochord lying between the neural tube 

 and the alimentary canal. In metamorphosis the tail is lost, 

 and with it the notochord, while the central nervous system is 

 reduced to a ganglion embedded in the body wall between the 

 two body apertures. 



Aside from such special instances where embryology sheds light 

 on problems of classification and therefore on problems of evolu- 

 tionary origins, there is in all of the Metazoa a common plan of 

 development followed by all of them. Thus the egg, a cell, is 

 the starting point in metazoan embryogeny. Cleavage, a period 

 of cell division following fertilization of the egg, occurs in all 

 Metazoa, and leads in turn to the formation of the blastula, 

 gastrula, and differentiation of the germ layers. Coelenterates 

 go very little beyond the development of two germ layers. 

 Hydra in its adult stage is but little more than a tube of ecto- 

 derm, lined with endoderm, with just the beginning of a trace of 

 a third germ layer, represented by the mesoglea, between them. 

 It might be said that the development of Hydra is arrested at the 

 two-germ-layer stage. Higher forms, on the other hand, pass 

 through and beyond the diploblastic condition and add a third 

 layer, the mesoderm; the three germ layers serve as the point of 

 departure for the differentiation of the tissues and organs of the 

 embryo and later of the adult. Gastrulation is a fundamental 

 process in the development of all Metazoa. 



One of the characters distinguishing Vertebrata from other 

 subphyla of the Chordata is the presence of the vertebral column, 

 which in its embryonic origin and general development is the 

 same in all of them. Since similarity in embryonic origin of the 



