48 AUGUSTA ÄRNBÄCK-CHRISTIE-LINDE, NORTHERN AND ARCTIC INVERTEBRATES. 



of the external layers of the stomach-wall and should be subject to a histological 

 investigation. In most Styelids, at least in preserved specimens, the longitudinal 

 folds are well visible, the external layers being thin and transparent; in tuberculata 

 they seem to be of a greater thickness. Nevertheless this difference can hardly be 

 regarded as a valuable species character, especially as — which will be shown in 

 the following description of the variety groenlandica — an externally smooth sto- 

 mach-wall occurs in other forms also. 



Ritter's species subpedunculata was distinguished also by characters exhibited 

 by the branchial sac, characters which, in all Styelids, generally seem to be of a 

 relative value only. Even in individuals belonging to the typical form the number 

 of the internal longitudinal vessels and the branchial folds show a very high degree of 

 variability (ef. Hartmeyer 1. c). In young ones I have often found one single 

 intermediate vessel, sometimes no such vessel, between some pair of folds. 



In well-developed specimens the second and the fourth folds are very often 

 reduced as compared with the two others; in young ones the difference is striking: 

 I have observed, in a few cases, only three branchial folds on the left side, the 

 second having been reduced. The retaining of tuberculata and subpedunculata as 

 species is thus impossible, the internal features used in distinguishing them occurring 

 within the range of variations of the forma typica. 



Hartmeyer's species kiikenthali should be characterized by a two- or three- 

 branched gonad, the reduced number of vessels and folds of the branchial sac, the 

 globular form of the body, thus, features of dubious value as far as this species is 

 concerned. Moreover, those characters are often found in young ones of aggregata, 

 why one might suppose that forms described under the name of kiikenthali are young 

 ones of the last-mentioned species. Or they might represent the solitary type of 

 that species. D. aggregata occurs, as is known, both solitary and aggregated into 

 groups and it is most probable that the two forms may differ in certain respects. 



The variety pulchella, re-described by Van Name (1. c), is no doubt identical 

 with D. kiikenthali, the different number of the intermediate vessels being the only 

 known difference. As the name pulchella has priority, this name is used in the 

 present paper. 



According to Ritter (1. c. p. 485) the species adolphi is distinguished from 

 aggregata by the greater number of tentacles, »a difference too great to be due to 

 fluetuating variation». As is well known, the number of tentacles is subject to 

 individual variation, and it has often been observed that they are of a less number 

 in young individuals than in full-grown ones. It is thus evident that one cannot 

 give much weight to this character for distinguishing a species. Other features re- 

 garded by the above-mentioned author as peculiar to this form, have been found 

 again in the variable typical aggregata. 



Judging from the description given by Bjerkan the species cylindrica seems 

 to represent a young one of aggregata. The four-branched gonad, the condition of 

 the branchial sac, the small number of the tentacles seem to support this view. 



The form described in the present paper under the name of groenlandica re- 



