94 SUMMARY OF CURRENT RESEARCHES RELATING TO 



considered as that of the male duct is the opening of the sucker, and that 

 which has heen taken for the female orifice is that of a large cavity which 

 is in relation to a large glandular mass. The digestive apparatus extends 

 through the whole of the posterior cylindrical region, and the excretory 

 apparatus is well developed. Save for the fact that the suckers are dorsal, 

 PolycoU/le ornnta has all the characters of a Diplostoraid, and should be 

 placed in the family of the Diplostomidae, and not, as Suhm thought, near 

 the Polystomidse. 



Nemerteans of Roscoff.* — In Angust 1885 M. F. Chapuis collected at 

 Eoscoff thirty-five species of Nemerteans, among which Cephalothrix viridis, 

 Polia caeca, Lineus variegatns, Cerebratulns modestus are new species ; there 

 is also a new variety of Cerebratulns fasciatus with the sides and lower 

 surface white. Tetrastemma diadema diflfers in colour from the specimens 

 described by Hubrecht. 



Ova and Development of Rotatoria.| — Herr G. Tessin commences his 

 essay with some notes on the female generative apparatus, and the forma- 

 tion and maturation of the ova, in which the observations of Plate aro 

 discussed and criticized ; the somewhat irregular segmentation is next 

 described, and gastrulation is found to be epibolic. The mesoderm appa- 

 rently, but not really, arises from the ectoderm, and commences to be 

 formed at the anterior end ; the observations of Zacharias are traversed. 

 The ectodermal cells are for a long time remarkable for the difference 

 between the dorsal and primitively ventral cells ; the cephalic region is 

 exclusively formed of the small primitively dorsal cells, while the ectoderm 

 of the trunk and tail is derived from three ventral ectodermal cells ; what 

 Salensky regarded as the central organ of the nervous system really gives 

 rise to the pharynx and the wheel-organ. The first part of the nervous 

 system to appear is the eye- spot, which marks the position of the brain. 

 The endoderm has early the appearance of one small posterior and two 

 larger anterior cells. 



With regard to the systematic position of the Eotatoria, as to which so 

 many very different propositions have been maintained, Herr Tessin re- 

 marks that they all agree in considering the adult organism to the ex- 

 clusion of its mode of development. The peculiar mode of segmentation, 

 and the fact that a part of the ectoderm long remains connected with the 

 endoderm, while the mesoderm is early separated from the latter and con- 

 nected with the ectoderm, appears to be a secondary change ; the difference 

 between them and other Bilateralia in the mode of origin of the mesoderm 

 is only apparent. Gastrulation results in the appearance of a hypogastric 

 bilateral form ; the prostoma does not pass to one pole of the egg, but to 

 what will be the ventral surface, and it marks the place of the definite 

 mouth, which in all hypogastric Bilateralia arises either directly or indi- 

 rectly from the prostoma. The further characteristic — of a transverse axis 

 — is also developed. It seems, indeed, to be certain that the Rotatoria 

 must be regarded as true hypogastric Bilateralia. 



With regard to their relationship to the Annelida — a view which has 

 been based by Hatschek on his well-known trochophore stage — the author 

 objects to the homology instituted between the oral circlets of cilia in 

 Eotatoria and the preoral and postoral ciliated circlets of the trochophore. 

 When the development of the wheel-organ is studied, and its origin from 

 an anterior ectodermic invagination (just like the tentacles of the Bryozoa) 



* Arch. Zool. Expe'r. et Gen., iv. (1886) pp. xxi.-iv. 



t Zeitschr. f. Wiss. Zool., xliv. (1886) pp. 273-302 (2 pis.). 



