282 Transactions of the Society. 



This bright, common, and widely distributed species has been 

 figured best by Dr. Hudson in his monograph, fig. 2, pi. XIII. It 

 was probably seen by most observers in the early days of Micro- 

 scopy, but Ehrenberg was the first to figure and describe it with 

 sufficient accuracy for future identification. 



In the early spring, it is, as a rule, the most abundant Eotifer 

 in nearly all lakes and pools ; it is also frequently met with 

 throughout the summer and autumn, and even in winter under 

 the ice many inches thick. Its ubiquitous habitat is made evident 

 by its presence in nearly every list of Eotifers that has been pub- 

 lished in England as well as on the Continent of Europe, in 

 America, Australia, and South Africa. I cannot, however, help 

 expressing a suspicion that one or two other species, particularly 

 S. oblonga, have sometimes been mistaken for tremula. 



In size it is decidedly smaller than 8. pectinata with which it 

 is frequently associated. In colour it often has a slight yellowish 

 tinge which contrasts with the white transparency of S. pectinata 

 when seen together. The integument is thin, white, and trans- 

 parent, and has some slight longitudinal folds along the dorsal 

 side. 



In shape the body of S. tremula is that of a slender cone, quite- 

 straight and flat in front, the small rounded auricles forming a 

 lateral prolongation of the flat frontal surface. In young animals 

 the sides of the body are quite straight also, but in well-fed speci- 

 mens the sides bulge out more or less. The foot is short, less wide 

 in girth than the apex of the body, and tapers to the two small 

 acute toes. 



The front of the head bears two pairs of styles, the larger outer 

 pair arise from very small triangular fleshy flaps. Two pairs of 

 setose pimples are, as usual, situated ventrally on each side of the 

 mouth, each bearing two or three stiff divergent hairs. The ciliary 

 wreath consists of a nearly straight band along the dorsal border 

 of the head, which, however, is interrupted by a dorsal gap in the 

 middle, and ventrally by two strongly ciliated cushions, one on 

 each side of the mouth. 



The auricles are thin, small, semicircular, of usual structure, 

 and in line with the front of the head. 



The mouth is situated on the ventral half of the head ; it is 

 shield-shaped, and surrounded by the usual single row of very 

 small stiff hairs curving over it all round. 



The dorsal antenna protrudes from an eminence in the usual 

 position above the eye and is connected by two rocket-shaped en- 

 largements and nerve-threads with the brain. The lateral antennae 

 are found very low down at the sides of the body, close above 

 the foot. 



The eye, seated on a granular brain-sac, is deep red, rounded 

 and of usual structure ; the red granules do not always fill up the 



