1883.] Mr Sedgwick, On the origin of segmented animals, etc. G3 



Facts of Coelenterate Ana- 

 tomy. 

 1. 



Elongated mouth of Actinozoa, 

 and the differentiation of that 

 mouth into two parts 1 . 



The special aggregation of the 

 nervous system round that mouth 

 (Medusce, Actinozooid Polyps). 



Bilateral symmetry of Acti- 

 nozoa. 



Facts from development of 



Triploblastica. 



1. 



The embryonic development of 

 Peripatus Capensis 2 . 



The slit-like form of the blasto- 

 pore in several animals (first point- 

 ed out by Lankester in Mollusca). 



The peculiar behaviour of the 

 blastopore in several animals re- 

 ceives light from this hypothesis. 



The arrangement of the central 

 nervous system in the Triplo- 

 blastica. 



2. 



The Ccelenterata present a serial 

 repetition of certain organs similar 

 to that found in metamerically 

 segmented animals ; and just as in 

 the latter the repetition corre- 

 sponds in each case to the meso- 

 blastic somites, so in the former 

 it corresponds with that of the 

 alimentary pouches. Thus for each 

 mesoblastic somite we find typi- 

 cally in segmented animals a nerve 

 ganglion, a nephridium, a segment 

 of the muscular system, and an 

 external appendage; while for each 

 pouch in a Medusa we find a sense 

 organ or ganglion or both, an ex- 

 cretory pore (Hertwig loc. cit), a 

 segment of the muscular system 

 (circular striated muscles of sub- 

 urnbrella of Medusa broken up 

 into segments by each radial 

 pouch) and a tentacular organ. 



2. 



The development of the body 

 cavity from arch enteric pouches 

 in Brachiopoda, Sagitta, Balano- 

 glossus ; and particularly in Am- 

 phioxus in which each somite (at 

 least the 14 anterior pairs) is 

 derived from a distinct pouch. 



1 Vide Hertwig, "Die Actinien," and Hickson, "On the Ciliatel Groove 

 (Siphonoglypke) in the StoniodEeuni of Alcyonarians," Proc. Roy. Soc. no. 226, 1883. 



2 Balfour, "Anatomy and development of Peripatus Capensis," Quart. J. of 

 Mic. Sci. April, 1883. As Dry. Kennel has chosen to throw some doubt upon the 

 correctness of the observations recorded in this paper, I will take this opportunity 

 of stating that the facts enumerated on pp. 256, 257 which Professor Mosely and 

 myself regarded as established by the investigation of the embryos then at our dis- 

 posal, are perfectly correct. I may perhaps be permitted to point out to Dr v. 

 Kennel a fact which he has apparently overlooked, viz. that he is working at 

 P. Edwardsii, a species differing considerably from P. Cajiensis. 



