106 



ON SOME ARCTIC NEMBRTBANS. 



[May 21, 



The foregoing account differs in several points from that given 

 by Burger ^ for two other species of the genus, viz. D. crassus and 

 D. cerinus. In the former species, whilst the ova are still quite 

 small, an oval deeply staining body is found lying in close proximity 

 to each of them. These subsequently increase in number, surround 

 the ovum, and later become metamorphosed into yolk. At a 

 later stage, when the ova are already of considerable size, there is 

 developed around them a delicate follicular tissue like a network, 

 in the meshes of which the ova then lie. What the nature of the 

 coverings of the ova are, and whether the follicular epithelium 

 plays any part in their formation, Biirger does not state. 



The process which occurs in D. horealis throws some light upon 

 tlie peculiar nuclear-like little bodies which give rise to the yolk 

 in D. crassus, and which have also been found by Hubrecht" in 

 Amphiporus marioni. They are probably to be regarded as 

 primitive ova in which the nucleolus has greatly increased in size 

 at the expense of the rest of the cell, and it seems plausible to 

 regard the enormous development of the nucleolus at the expense 

 of the rest of such primitive ova as a stage in their conversion 

 into yolk. The peculiar behaviour of the follicle-cells in D. horealis 

 is probably correlated with the relatively great size attained by 

 the ova in this species. Whilst in D. crassus the nearly ripe ova 

 measure 144 ju x 100 jw,, in D. ho^-ealis thev attain the dimensions 

 of 700/ix420/x. 



Gatty Marine Laboratory, 

 St. Andrews, N.B. 



EXPLANATION OF THE PLATES. 



Lettering in both Plates. 



bm., basement-membrane. 

 cc, ciliated canal of cerebral 



oi'gan. 

 cl., cephalic vascular loop. 

 da., cephalic lacuna. 

 coTff., cerebral organ. 

 cts., spaces in gelatinous connective 



tissue. 

 dbv., median dorsal blood-vessel. 

 dc, dorsal nervous commissure. 

 dg., dorsal ganglion. 

 ep., epithelium. 

 exd., excretory duct. 

 ext., excretory tubules. 

 fr., frontal organ. 

 get., gelatinous connective tissue. 

 glcorg., glands of cerebral organ. 

 gll., lateral glands. 

 gs., gonidial sac. 



hs., head-slit. 

 Ibv., lateral blood-vessel. 

 II., lateral blood-lac ana. 

 m., mouth. 



■mc., circular muscle-layer. 

 mcc., circular muscle-layer of cutis. 

 ml., longitudinal muscle-layer. 

 mlc., longitudinal muscle-layer of 



cutis. 

 mlo., outer longitudinal muscle- 

 layer. 

 (US., oesophagus. 

 ps., proboscis-sheath. 

 rd., rhynchocoelomic diverticulum. 

 rde., expansioiis of rhynchocoelomic 

 diverticula. 

 ss., nervous side-stem. 

 vc, ventral brain commissure. 

 vg., ventral ganglion. 



» marks point vrhere the median blood-vessel leaves the proboscis-sheath. 



1 Zeitsch. fiir wiss. Zool. 50 Bd. 1890, p. 243, and pi. x. 

 ^ ' Challenger ' Reports, vol. ix. (the Nemertea) p. 120. 



