318 GENERAL CONCLUSIONS. 



the characters of the larval prototype in the course of its conversion 

 into a bilateral form. 



5. The Trochosphere is a completely differentiated bilateral form, 

 in which an anus has become developed. The praeoral ciliated ring 

 of the Trochosphere is probably directly derived from the ciliated 

 ring of Pilidium, which is itself the original ring of the prototype of 

 all these larval forms. 



(5. Echinoderm larvae, in the absence of a nerve-ganglion or 

 special organs of sense on the praeoral lobe, and in the presence of 

 alimentary diverticula, which give rise to the body cavity, retain some 

 characters of the prototype larva which have been lost in Pilidium. 

 The ciliated ring of Echinoderm larvae is probably derived directly 

 from that of the prototype by the formation of an anus on the dorsal 

 side of the ring. The anus was very probably originally situated at 

 the aboral apex. 



Adult Echinoderms have probably retained the radial symmetry 

 of the forms from which they are descended, .their nervous ring being 

 directly derived from the circular nervous ring of their ancestors. 

 They have not, as is usually supposed, secondarily acquired their 

 radial symmetry. The bilateral symmetry of the larva is, on this 

 view, secondary, like that of so many Coelenterate larvae. 



7. The points of similarity between Tornaria and (1) the Trocho- 

 sphere and (2) the Echinoderm larvae are probably adaptive in the 

 one case or the other ; and, while there is no difficulty in believing 

 that those to the Trochosphere are adaptive, the presence of a 

 water- vascular vesicle with a dorsal pore renders probable a real 

 affinity with Echinoderm larvae. 



8. It is not possible in the present state of our knowledge to 

 decide how far the resemblances between Actinotrocha and Echino- 

 derm larvae are adaptive or primary. 



BIBLIOGRAPHY. 



(257) Allen Thomson. British Association Address, 1877. 



(258) A. Agassiz. "Embryology of the Ctenophorffi." Mem. Amer. Acad. of 

 Arts and Sciences, Vol. x. 1874. 



(259) K. E. von Baer. Ueb. Entwicklunnsgescliichte d. Tliiere. Konigsberg, 

 18281837. 



(260) F. M. Balfour. "A Comparison of the Early Stages in the Development 

 of Vertebrates." Quart. Joiirn. of Micr. ,SV/., Vol. xv. 1875. 



(261) C. Claus. Die Typcnh'hre u. E Haeckrl'* sr/. Gfistrffa-theorie. Wien, 

 1874. 



(262) C. Claus. Grundziige d. Zoolonie. Marburg und Leipzig, 1879. 



(263) A. Dolirn. Der Urspruny d. Wirbelthiere it. d. Princlp di's Ftuictioiis- 

 icechsels. Leipzig, 1875. 



(264) C. Gegenbaur. Grundriss d. rerijleichendcn Anatomic. Leipzig, 1878. 

 Vide also Translation. Elements of Comparative Anatomy. Macmillan & Co. 1878. 



(265) A. Gbtte. Entioicklungsgeschichte d. Unke. Leipzig, 1874. 



(266) E. Haeckel. Studien z. Gastreea-theorie, Jena, 1877; and also Jenaische 

 Zeitschrift, Vols. vin. and ix. 1874-5. 



(267) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation. 

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