650 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The ectoderm of the larva in the neck region and at the front end 

 of the fins is thickened and consists of vacuolated cells like those com- 

 posing the epidermis of Spadella draco. No trace of excretory organs 

 nor of genital ducts was found in the larva ; the latter appears only as 

 maturity approaches. There is no ccelomic epithelium, but the muscles 

 are formed from the basal ends of the cells which line the coelom, as in 

 Nematodes. This fact, combined with the mode of origin of the trans- 

 verse septa and the absence of many Annelid characters, supports the 

 view that the Chaetognatha are not related to the Annelida. 



Jn a subsequent paper * L. Doncaster describes the complete and 

 equal cleavage, the blastula with its nuclei at the outer ends of its 

 cells, the gastrula arising by invagination, and the early appearance of 

 the primitive genital cells (at first 2, then by division 4), which are 

 separated from the inner layer of the gastrula at the opposite pole from 

 the blastopore. Soon after their appearance a pair of folds arise at the 

 anterior pole, which, growing backward, divide the archenteric cavity 

 into three divisions, — a median alimentary canal and two lateral coelomic 

 cavities. 



The folds push before them the genital cells for some distance, and 

 the latter ultimately come to rest in the ccelomic cavities, two in each 

 •side. At about this stage the front ends of the coelomic spaces become 

 separated off as head-cavities, and the ectoderm, invaginating till it 

 meets and fuses with the endoderm, forms the mouth. The blastopore 

 comes to lie a little in front of the posterior end before it closes, and 

 by the lengthening of the embryo in the shell and the general com- 

 pression caused thereby, all cavities become obliterated. The origin 

 of the cerebral and ventral ganglia by ectodermic thickening and in- 

 sinking is then described, as well as the four longitudinal aggregations 

 of the mesoderm. 



The larva, which escapes from the shell, usually not more than two 

 days after the eggs are laid, is a little tapering rod about 1 mm. in 

 length, solid and without cavities. Doncaster describes the differentia- 

 tion of the ganglia, the musculature, the ccelomic cavities, the gut, and 

 so on, till the definitive structure is reached. 



" In the present state of our knowledge it seems safest to regard 

 the Choetognatha as descended from a primitive ccelomate stock, from 

 which the Annelida have arisen on the one hand; while, on the other, 

 the Nematoda probably branched off, but lost many of their original 

 characters owing to their parasitic habit." 



Nematohelmmthes. 



Largest of known Nematodes. f — J. F. Gemmill and 0. von Lin- 

 stow describe Ichthyonema grayi sp. n., found by Gemmill in the body- 

 cavity of Echinus esculentus from the Firth of Clyde, and also by 

 Shipley at Plymouth. The female measured 1500 mm., the male 

 60 mm. in length. The authors compare the new form with the other 

 (7) species of the genus. 



* Quart. Journ. Micr. Sci., xlvi. (1902) pp. 351-98 (3 pis.). 

 t Arch. Naturges., Ixviii. (1902) pp. 113-8 (1 pi.). 



