SEMPER 279 



animal kingdom, for they saw in this a strong argument for 

 the monophyletic descent of all animals from one common 

 ancestral form. 



In his first paper Semper does little more than break 

 ground ; he insists on the fact that both Annelids and 

 Vertebrates are segmented animals, and he points out how 

 close is the analogy between the nephridia or " segmental 

 organs" of the former and the excretory (rnesonephric) 

 tubules of the latter, upon which he published in the same 

 volume an extensive memoir. At this time he considered 

 Balanogtossus by reason of its gill-slits (its notochord he 

 did not know) to be the nearest living representative of the 

 ancestral form of Vertebrates and Annelida. 



His second paper is a more exhaustive piece of work 

 and deals with every aspect of the problem, both from an 

 anatomical and from an embryological standpoint. It is 

 consciously and admittedly an attempt to apply Geoffrey's 

 principle of the unity of plan and composition to the three 

 great metameric groups, the Annelida, Arthropoda, and 

 Vertebrata. Semper follows Geoffrey's lead very closely in 

 maintaining that it is not the position of the organs relative 

 to the ground that must be taken into account in establishing 

 their homologies, but solely their spatial relations one to 

 another. He holds that dorsum and venter are terms of 

 purely physiological import, and he proposes to substitute for 

 them the terms neural and cardial (better, haemal) surfaces, 

 either of which may be either dorsal or ventral in position. 



Having established this primary principle, Semper has 

 little difficulty in showing that the main organs of the body 

 lie to one another in the same relative positions in Annelida, 

 Arthropoda, and Vertebrata ; and this, together with the 

 metameric segmentation common to them all, constitutes 

 his first great argument in favour of their genetic relationship. 

 But he has still to show that Annelids possess at least the 

 rudiments of certain organs which seem to be peculiar to 

 Vertebrates, as the gill-slits, the notochord, and a nervous 

 system developed from the ectoderm of the " dorsal " surface. 

 He takes particular cognisance also of the old distinction 

 drawn by von Baer, that Vertebrates show a " double- 

 symmetrical " mode of development (evolutio bigeuiina], the 



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