SCIENCE^GOSSIP. 



121 



STRUCTURAL FEATURES IN AMERICAN ROTIFERA. 



By Dr. Alfred C. Stokes. 



THE following notes refer exclusively to certain 

 structural points which seem to have been 

 overlooked by other observers who have not 

 had these particular species in circumstances so 

 favourable as those in which they have presented 

 themselves to the writer. 



Brachionus bakeri, Ehr. 



In this country we have two forms of this 

 well-known rotiferon, neither of which can claim 

 even the dignity of a variety, as they differ from 

 the type only in size, a common occurrence with 

 the Rotifera in general, which, 

 in regard to dimensions as 

 well as to other more impor- 

 tant characters, are among the 

 most variable of microscopic 

 creatures. The British animal 

 is described in size as follows, 

 in comparison with the two 

 American forms of the same 

 species : British form — length, 

 ^ inch ; width, j-^j^ inch. 

 American form — length, -^g 

 inch; width, ^i^ inch. Ameri- 

 can form — length, j-J^ inch ; 

 width, Y^u inch. 



I have had both the American 

 representatives appear in great 

 numbers in an aquarium, fre- 

 quently finding the small and 

 the large forms on the slide 

 at the same time ; but while 

 these do not vary in structure, 

 I have observed with them one 



or two features not described as occurring with the 

 species as known in England and elsewhere in 

 Europe. 



Within the stomach, opposite the entrance of 

 the oesophagus and apparently continuous with 

 it, is an appendage previously undescribed and 

 presumably not observed in any other rotiferon, 

 except in certain American species (i). On ordinary 

 occasions this organ appears to be a conspicuous 

 membrane with a conve.x, free margin, the whole 

 vibrating in horizontal undulations, and measuring 

 about tjit'otj inch in width. This apparently mem- 

 brani-form appendage is perhaps more readily 

 seen through the dorsal aspect and less easily 

 through the ventral, but it is observable at all 

 times, either actually or by its effects ; even when 

 the stomach is filled with food the undulations may 



0) Ann. and Mag. Nat. Hist., July, 1896 ; Journ. Roy. Micr. 

 Sec, June, 1896. 



be indistinctly noted. After the rotiferon has 

 been starved for several hours (for from thirty to 

 thirty-four in this instance), the membrani-form 

 aspect of the organ disappears, and it then seems 

 to be elongate-fusiform in general contour, and to 

 be attached by its extreme right-hand apex to the 

 internal wall of the stomach, its optical form then 

 simulating that of a three-fold spiral, or a three- 

 bladed screw-propeller. That there could be a rigid , 

 screw-like organ attached as described, and actually 

 rotating on its long axis, as this then appears to 

 do, is of course impossible. Such an appendage 

 would soon twist all vitality 

 out of its ligament, and speedily 

 set itself free, to be digested 

 with the other contents of the 

 viscus. All these interesting 

 and not inelegant appearances 

 are illusory. The undulating 

 organ exists, but it is net 

 a membrane. To solve the 

 problem and leave no doubt 

 in the observer's mind is no 

 easy microscopical task, but 

 I can recommend it to those 

 that have access to Brachionus 

 bakeri as one worth struggHng 

 with. I can venture to say 

 that before positively deciding 

 as to the character of the 

 appendage, the observer will 

 arrive at several decisions to 

 reject them all, and that he 

 will at the first trial retire 

 from the field discomfited, fcr 

 the rotiferon will have the better of the contest. 

 As the organ often seems to be finely striated, 

 one decision will be that it is a broad band 

 of delicate flagella extending across the front of 

 the stomach, in close proximity to the entrance 

 of the oesophagus, and undulating there, the 

 transverse folds thus produced by the apparently 

 synchronous movements of the pendent flagella 

 simulating a membrane, and producing those 

 optical illusions which the appendage offers freely 

 even to the most patient observer. 



The organ is actually a tubular extension of the 

 oesophagus, as may be seen, when, by great good 

 fortune, the free extremity is lifted up so that 

 the flattened lumen may be noted. It seems to be 

 finely ciliated internally. The tube is homologous 

 with that in the crop of Floscularia and in Apsiltts, 

 but it differs from these in being in constant and 

 rapid motion. 

 F 4 



Brachionus bakeri, Ehr. 



