100 INVERTEBKATA CHAP. 



in ontogeny, and when the food necessary for the embryo is stored 

 up within its cells in the form of yolk-grains, and has not to be 

 sought for outside, then the inbending can be replaced by solid pro- 

 liferation because the larval gut is no longer a functional organ. 



We are thus led to form the following conception of the past history 

 of the lower Metazoa. A widespread and dominant race of blastula- 

 like animals once swarmed in the primeval seas. Some of these 

 took a creeping life and eventually gave rise to the group of sponges ; 

 others kept to the free-swimming life and developed into plaimlae, 

 and so gave rise to the Coelenterata. Some of these planulae, by the 

 specialization of the cilia into comb-like locomotor organs, became 

 Ctenophora ; whilst the remainder adopted a fixed life and attached 

 themselves by their aboral poles. This change occurred in different 

 divisions of the stock at different stages of the evolution of the 

 internal organs of the planula ancestor, and in this way the groups 

 of Hydrozoa, Scyphozoa, and Actinozoa arose. 



LITERATURE REFERRED TO 



Allman. A Monograph of the Gymnoblastic or Tubularian Hydroids. Ray Society, 

 1871 and 1872. 



Appellof. Studien iiber Actinien-Entwicklung. Bergens Museums Aarbog, 1900. 



Brauer. Uber die Entstehung der Geschlechtsprodukte und die Entwicklung von 

 Tubularia mesembryanthemum. Zeit. fur wiss. Zool., 1891. 



Carlgren. Actinien-Larven. Nordisches Plankton, 5 te Lieferung, 1906. 



Gary. The Formation of the Germ-layers in Actinia bermudensis. Biol. Bull. 

 (Wood's Hole), vol. 19, 1910. 



Chun. Fauna und Flora des Golfes von Neapel. Ctenophora, 1880. 



Chun. Zur Morphologie der Siphon ophoren. Zool. Anz., vol. 10, 1887. 



Dalyell. Rare and Remarkable Animals of Scotland. London, 1847. 



Driesch and Morgan. Zur Analyse der ersten Entwicklungsstadien des Cteno- 

 phoreneies. Arch. Ent. Mech., vol. 2, 1895. 



Duerden. West Indian Madreporarian polyps. Mem. Ac. Washington, vol. 8, 1902. 



Faurot. Developpement du pharynx, des couples et des paires de cloisons chez les 

 Hexactiniidae. Arch. Zool. Exp. (4 me serie), vol. 1, 1903. 



Fischel. Experimentelle Untersuchungen am Ctenophorenei, I. Arch. Ent. 

 Mech., vol. 6, 1897. Do. II. Arch. Ent. Mech., vol. 7, 1898. 



Friedemann. Untersuchungen Uber die postembryonale Entwicklung von Aurelia 

 aurita. Zeit. fiir wiss. Zool., vol. 71, 1902. 



Gardiner. FlabeUum, Anatomy and Development. Marine Invest. S. Africa. Cape 

 Town, 1902. 



Gotte. Vergleichende Entwicklungsgeschichte der Geschlechtsindividuen der 

 Hydropolypen. Zeit. fur wiss. Zool., vol. 87, 1907. 



Haeckel. System der Siphonophoren auf phylogenetischer Grundlage. Jen. Zeit. 

 Nat., vol. 22, 1888. 



Harm. Die Entwicklungsgeschichte von Clava squamata. Zeit. fur wiss. Zool., 

 vol. 73, 1903. 



Hein. Untersuchungen liber die Entwicklung von Aurelia aurita. Zeit. liir wiss. 

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Hein. Untersuchungen iiber die Entwicklung von Cotylorhiza tuberculata. Zeit. 

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Hickson. Alcyonium, Liverpool Mar. Biol. Com. Memoirs. No. 5, 1901. 



Huxley. Oceanic Hydrozoa. Publications of Ray Society, 1859. 



von Koch. Die Gorgoniden. Fauna und Flora des Golfes von Neapel, etc. No. 15, 

 1887. 



von Koch. Entwicklung von Caryophyllia cyathus. Mitt. Zool. St. zu Neapel, 

 vol. 12, 1897. 



