DEVELOPMENT. 197 



great divisions of the Animal Kingdom, and collect all the facts 

 which seem to be significant. We may omit the Protozoa, 

 which never develop multicellular tissues, and in which seg- 

 mentation and all subsequent development are therefore 

 absent. 



PORIFERA (Sponges). Nearly all marine and undergoing 

 metamorphosis, the larva being wholly or partial!}^ ciliated. 



CCELENTERATES undergo metamorphosis, the immediate pro- 

 duct of the ovum being nearly always ap/amifa, or two-layered 

 hollow sac, usually devoid of a mouth, arid moving about by 

 external cilia. In many Coelenterates the complicated process 

 of development known as Alternation of Generations occurs. 

 The sedentary Anemones pass through a planula stage, but 

 within the body of the parent. Among the few Ccelenterates 

 which have no free planula stage is the one truly fluviatile 

 genus Hydra. 



WORMS are remarkable for the difference between closely 

 allied forms with respect to the presence or absence of meta- 

 morphosis. The iion-paras4tic freshwater and terrestrial 

 Worms, however (e.g., Earthworms, Leeches, all freshwater 

 Dendrocoela, and Rhabdocoela), do not undergo metamorphosis. 

 In the parasitic forms complicated metamorphosis is common, 

 and may be explained by the extraordinary difficulties often 

 encountered in gaining access to the body of a new host. 



All POLYZOA are aquatic (fluviatile or marine), and all produce 

 ciliated embryos, unlike the parent. 



BRACHIOPODA are all marine, and produce ciliated embryos. 



ECHINODERMS usually undergo striking metamorphosis, but 

 certain viviparous or marsupial forms develop directly. There 

 are no fluviatile or terrestrial Echinoderms. 



LAMELLIBRANCHIATE MOLLUSCA have peculiar locomotive 

 larvae, provided with a ring of cilia, and usually with a long 

 vibratile lash. These temporary organs are reduced or sup- 

 pressed in the freshwater forms. There are no terrestrial 

 Lamellibranchs. 



SNAILS have also a temporary ciliated band, but in the fresh- 

 water species it is slightly developed (Limnceus), and it is totally 

 wanting in the terrestrial HeUcidce. 



