INTERPRETATION OF ONTOGENY 



277 



obscure its genetic affinities, which may still be clearly indicated 

 by embryonic or larval stages. 



The common Ascidian, for example (Fig. 129), is a sac-shaped 

 organism, permanently attached to rock or seaweed in the adult 

 condition, which by the uninitiated would hardly be taken for an 

 animal at all. Before their life-history was known the Ascidians 

 were placed by zoologists amongst the " Molluscoida," a group of 

 invertebrates supposed to resemble the mollusca or shell-fish. It 

 was only the discovery that 

 the typical Ascidians pass 

 through an active tadpole 

 stage in their development 

 (Fig. 130), with a muscular 

 tail, a notochord and a central 

 nervous system all formed as 

 in vertebrates, that demon- 

 strated their true position 

 as degenerate members of 

 the great phylum Chordata 

 (= Vertebrata in the widest 

 sense of the term), the tad- 

 pole stage being, of course, 

 . a recapitulation of the ances- 

 tral, fish-like, chordate con- 

 dition. 



It will be readily under- 

 stood from the foregoing 

 illustrations that in endea- 

 vouring to reconstruct the 

 ancestral history of an organ- 

 ism from the stages which it 

 passes through in its indivi- 

 dual development, we have to distinguish very carefully between 

 two sets of characters. In the first place there are characters which 

 are due to inheritance from the adult condition of very remote 

 ancestors and which really indicate stages in what we may call 

 the direct line of evolution. These are termed palingenetic or 

 ancient characters. Such, for example, are the essential features 

 of the gastrula stage, which occurs so generally and which indi- 

 cates a remote ccelenterate ancestor (the Gastrtea of Professor 

 Haeckel) for at any rate the great majority of multicellular animals. 



FIG. 128. The Zosea Larva of a 

 Crab (Portunus), X 30. (From a 

 photograph.) 



