The Genus Synchwta. By C. F. Eousselet. 285 



toes two, small, acute, well separated. Size up to 225 fz ( T {^ in.) 

 in length by 115 fi (^ n in.) wide at the auricles. Male 102 /* 

 (s^ff m -) m l en g tn - Lacustrine. 



Although a very common species everywhere in fresh-water 

 ponds and lakes, I fear this animal has not been clearly recognised 

 until quite lately. One reason possibly is that Ehrenberg gives it 

 a size equal to that of S. peeUnata, which is obviously not correct ; 

 another reason is that Mr. Gosse has figured under this name of 

 oblong a an animal which certainly was a sick pectinata, the two 

 frontal fleshy little horns being quite sufficient to recognise it 

 as such. 



Most workers will have taken this common Synchseta for a 

 small S. tremula, whilst I have for a considerable time past sepa- 

 rated it from tremula, but have confounded it with S. gyrina of 

 Hood, which I now know is a larger and exclusively marine and 

 brackish water species. In the lists of Eotifers, therefore, found 

 at the Quekett Club's excursions published in the Quekett Journal, 

 wherever S. gyrina is mentioned, this name should be altered into 

 oblonga. Quite recently Dr. Zacharias, recognising its distinctive 

 characters, has given it the new name of S. neglccta, while it ia 

 represented in his earlier lists under the name of S. tremula. 



Prof. Ehrenberg in his great work describes S. oblonga as the 

 commonest Synchseta occurring in his time near Berlin, and 

 usually associated with pectinata and tremula. This is exactly 

 what I find to be the case here near London at the present time. 



At first sight, and when observed with a low power, it has 

 much resemblance in shape and size with a small S. tremula. A 

 closer acquaintance, however, shows a number of differences in 

 shape, structure, and habits, which can always be readily recog- 

 nised. Perhaps it will be convenient if I first clearly state the 

 differences which separate these two species. >S y . tremula has a 

 top-shaped body with the front of the head quite straight and flat, 

 and the auricles in a line with the front. S. oblonga, whilst also 

 more or less cone-shaped, is, as a rule, more swollen at the sides 

 and has the front of the head convex, and the auricles are lower 

 down at the end of the convex frontal curve. The lateral antennae 

 in & tremula are situated low down near the base of the foot ; in 

 oblonga these organs are much higher up at the sides of the body 

 and slightly ventral in position. On the extreme front of the 

 head, when seen from the dorsal side, S. oblonga has two divergent 

 tufts of very fine stiff sense-hairs which are absent in tremula. 

 Finally, & tremula is a little, but distinctly, larger in size and 

 more yellowish in colour ; it also likes to anchor itself on a thread 

 from the toes and spin round and round on its longer axis on the 

 spot ; this habit S. oblonga does not possess. The two figs. 2 

 and 3 on pi. III., drawn from life by Mr. Dixon-Nuttall, will give 

 clear idea of these differences. 



June 18th, 1902 u 



