100 INVEETEBEATA 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 planulae, 

 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 Gyrunoblastic or Tubularian Hydroids. Ray Society, 

 1871 and 1872. 



Appellbf. Studien iiber Actinieii-Entwickhmg. Bergens Museums Aarbog, 1900. 



Brauer. Uber die Entstehung der Geschlechtsprodukte mid die Entwicklung von 

 Tubular la mescmbryanthemum. Zeit. fiir 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 Morphologic der Siphonophoren. 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. Eut. Mech., vol. 2, 1895. 



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



Faurot. Developpement clu pharynx, des couples et des paires de cloisons die/ les 

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



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

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



Friedemann. Untersuchungen iiber die postembryonale Entwicklung von Aurelia 

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



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

 Town. 1902. 



Gotte. Vergleichende Entwicklungsgeschichte der Geschlechtsindividuen der 

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



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

 Nat., vol. 22, 1888. 



Harm. Die Entwicklungsgeschichte von Clara squamata. Zeit. fiir wiss. Zool., 

 vol. 73, 1903. 



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

 Zool., vol. 67, 1900. 



Hein. Untersuchungen iiber die Entwicklung von C'o/i/JnrJ/t'-a tubcrculata. Zeit. 

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Hickson. Alciioninm, 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 Caryqphyllia cyathits. Mitt. Zool. St. zu Neapel, 

 vol. 12, 1897. 



