PROTOZOA AND METAZOA. 249 



Gastrula, or intestinal larva, which recurs in a remarkably 

 similar form in the history of the individual development 

 of the several groups of animals, is of the greatest 

 significance. From this Gastrula the lowest Vertebrate 

 develops, just as the lower forms of Worms, Soft-bodied 

 Animals, Star-animals, Plant-animals, etc (Cf. Plates II., 

 Ill, and Fig. 22-28, pp. 191, 193.) The Gastrula at the 

 present day presents a correct picture of the primitive 

 Gastraea, which must have developed from the Protozoa in 

 the Laurentian period. 



Comparative Anatomy and Ontogeny teach us, further, 

 that from this Gastrsea the animal kingdom at first de- 

 veloped in two diverging directions or lines. In the one 

 direction proceeded the low group of the Plant-animals 

 (Zoophytes), to which the Sponges, Polyps, Corals, Medusae, 

 and many other marine animals belong ; and among fresh- 

 water animals the well-known Hydra, or fresh-water Polyp, 

 and the Spongilla, or fresh-water Sponge. In the other 

 direction, the very important group of the Worms, in the 

 narrower sense in which the present zoological classification 

 limits this group, developed from the Gastraea. In the 

 Linnaean system, and generally in earlier times, all the 

 lower animals, Infusoria, Worms, Soft-bodied Animals, 

 Plant-animals, Star-animals, etc., were included under the 

 name of Worms ; the name is now, however, much more 

 narrowly restricted to the true Worms. Under it are in- 

 cluded Earth-worms, Leeches, Ascidians, and also the 

 various parasitic Worms, Tape-worms, Round-worms, 

 Trichinae, etc. Different as all these worms appear, in their 

 perfect state, they can all be traced back to the Gastraea. 

 (Cf. Table XVIII. in Chap. XVII.) 



