Hyatt.] 108 [March 



in the Worms, Insects, Arthropoda and Vertebrata, arises through 

 transformations in which the vertical antimera of the Radiata 

 become converted into horizontal, bilateral somites. The the- 

 ory is founded largely upon the elongation of the actinostome or 

 mouth of the Actinozoa, and the differentiation observed between 

 the anterior and posterior ends. The primitive actinostome is 

 round in the young but elongated in the adults, and while the 

 central parts are approximate and more or less adherent, there 

 are tubes kept open at the ends, one of which is ciliated and used 

 as a water tube, while the other has excretory functions. Sedg- 

 wick supposes that this differentiation is parallel with that which 

 takes place in a more abbreviated form in the vertebrate embryos, 

 when the medullary plate, which he considers identical with, and 

 as an elongation of, the blastoporic area and primitively 

 ectoblastic, arises, becomes invaginated, and forms the neural 

 canal. The canal thus formed is supposed, when presented in 

 this general way, to be the homologue of the actinostome, and 

 -instances are cited of its tubular connection with the mesenteron 

 below in embryos and Ascidia. The posterior opening is 

 supposed to be identical with the blastopore and to be connected 

 with the formation of the anus, and an anterior opening as possi- 

 bly present and bearing a similar relation to the mouth of the 

 vertebrata. 



Prof. Sedgwick's hypothesis is fascinating and seems to be in 

 accord with the general homologies between Radiata and Articu- 

 lata, and also agrees fairly well with the mode of development of 

 the coelomic cavities, whose paired symmetry is hard to account 

 for on any other hypothesis. The author, however, insists upon 

 the view that the stomodeum and proctodeum are necessarily 

 derivatives of the elongated blastopore, whereas in sponges the 

 evidence seems to be against this view; and, also, in other types 

 in which the stomodeum arises as an independent invagination. 

 Sedgwick's results with regard to the coincidence of the blasto- 

 pore and the stomodeum and proctodeum have been denied by 

 Dr. Kennel (Arb. Zool. Zoot. Inst. Wurzburg., vol. vn, 1884) 

 after prolonged and profound investigations of the embryology 

 and histology of Peripatus Edwardsii and torquatus from the 

 West Indies. P. capensis, from Cape of Good Hope, was the 

 species used by Sedgwick, and the differences noted by Kennel 



