618 SUMMARY OF CURRENT RESEARCHES RELATING TO 



senting at least 28 species. Seventy-six were Entomostraca of 20 

 species. Nearly three-fourths were Oladocera. One-third were 

 Acrojjerus leucocephalus Koch. 



Vermes. 



Development of Annelids* — Professor W. Salensky gives 

 an account of his own observations ; in dealing with marine 

 forms, he studied among others a species of Terebella, Nereis 

 cultrifera, and Spio fuliginosus. In all that he studied he found 

 cleavage to be unequal, and to lead to the formation of an amphi- 

 gastrula ; before the division on the surface of the egg lively proto- 

 plasmic movements were always observed in Spio, the protoplasm 

 giving rise to lobate, clear, pseudopodia-like processes, which altered 

 in form during the whole process of division, and at its conclusion 

 were withdrawn. The first rudiment of the mesoblast appeared to 

 arise from the micromeres of the second cleavage ; in some other 

 cases the mesoderm appeared in the form of two primitive mesoblasts 

 lyinw near the edge of the blastopore ; in Psygmobranchus the 

 primitive endodermal cells do not represent the whole endoderm, for 

 they only form the dorsal portion of the nutrient cavity, while its 

 ventral wall arises from a collection of cells on the ventral surface 

 (secondary endoderm). The author finds that the mode of formation 

 of the mid-gut is different in the forms examined by him ; in Nereis 

 dumerilii Goette found that the endoderm arose from the four large 

 endodermal cells, in the form of a ventral cord of cells, while the 

 four (and later) five large fat-containing cells were pressed forward 

 and dorsally, and used as nutrient yolk. In this arrangement the 

 author found Psygmobranchus to agree more closely than N. cultrifera. 

 Differences were observed in the origin of the fore- and hind-guts, 

 which in Psygmobranchus and Aricia had an endodermal, and in Nereis 

 an ectodermal origin. Striking as this difference would appear to be, 

 it was found to be really more quantitative than qualitative ; and 

 the endodermal origin is to be regarded as nothing more than the 

 result of an incomplete ectodermal invagination. 



Greater similarities were noted in the mode of development of 

 the nervous system ; and the results of Kleinenberg and other later 

 investigators as to the independent origin of the supra-oosophageal 

 ganglion and the ventral ganglionic chain were confirmed. It was 

 noted that a cord-like process always extended from the lower surface 

 of the frontal plate to the subjacent mesoderm ; and these were 

 regarded as being homologous with the cords which, in vermian 

 larva? provided with mesenchymatous cells, form the rudiments of 

 these cells. After discussing the characters of the ventral ciliated 

 groove, Salensky states that in not very young larvae of Psygmo- 

 branchus he observed, between the epithelium of hind-gut and the 

 " Darmfaserblatt," a cavity filled with clear fluid; the wall of the 

 cavity was contractile and exhibited regular pulsations, by means of 

 which the fluid was driven forwards. In Terebella, likewise, the 

 formation of the blood-vessels is preceded by such a perigastric 



* Biol. Centralbl., ii. (1882) pp. 198-208. 



