DEVELOPMENT OF WORMS AND PLANT-ANIMALS. 73 



several comprehensive groups and also single isolated genera 

 of Worms are to be regarded as root-suckers which have 

 sprouted directly from the rest of the primaeval family-tree 

 of the Worms. Some of these suckers have evidently 

 changed but little from the long-extinct parent-form, the 

 Primitive Worm (Prothelmis), which is immediately con- 

 nected with the Gastrsea. 



Comparative Anatomy and Ontogeny clearly and sig- 

 nificantly prove that the Gastrsea must be regarded as 

 the direct ancestor of this Primitive Worm. Even now, a 

 gastrula develops from the egg of all Worms after its 

 cleavage. The lowest and most imperfect Worms retain 

 throughout life an organization so simple that they are but 

 little raised above the lowest Plant-animals, which are also 

 immediate descendants of the Gastrsea, and which also yet 

 develop directly from the gastrula. If the genealogical 

 relation of these two lower animal tribes, the Worms and 

 the Plant-animals, is closely examined, it becomes evident 

 that the most probable hypothesis of their descent is, that 

 the two originated, as independent branches, directly from 

 the Gastrsea. On the one side, the common parent-form of 

 the Worms developed from the Gastrsea ; as, on the other 

 side, did the common parent-form of the Plant-animals. 

 (Cf. Tables XVIII. and XIX.) 



The tribe of Plant-animals (Zoophytes, or Ccelenterata) 

 now comprehends, on the one side, the main class of Sponges 

 (Spongice) ; on the other, the main class of the Sea- nettles 

 {A calephce) ; to the former belong the Gastrseads and 

 Poriferse, to the latter the Hydroid-polyps, the Medusae, 

 Ctenophorse, and Corals. From the Comparative Anatomy 

 and the Ontogeny of these we may infer, with great pro- 



