222 THE AMERICAN MONTHLY [July 



A Spherical Rotifer. 



WITH FRONTISPIECE. 



A remarkable rotifer, Trochosphsera solstitialis, found 

 in the Yangtsze Kiang river at Wuhu in August, 1892, by 

 Surgeon Gunsbn Thorpe ; in the Illinois river in August, 

 1896, by Dr. C. A. Kofoid; was taken in a pond near Lake 

 Erie in 1898. The anatomy of the animal is extremely 

 simple and beautifully displayed, all the organs, usually 

 so indistinct and closely packed together in rotifers, being 

 here spread out and suspended in the transparent sphere 

 in the most delightful manner. The ciliary wreath en- 

 circles the sphere above the middle, leaving the usual 

 dorsal gap, and dividing it into two unequal segments, 

 the larger oral segment containing all the organs, and the 

 smaller aboral segment having nothing at all. Close be- 

 low the wreath on the ventral side is the mouth and mas- 

 tax (m); a long thin oesophagus leads into the alimentary 

 canal, which is suspended in the centre of the sphere. 

 This canal has about its middle a single small knob-like 

 outgrowth or gland, makes half a corkscrew turn, and ends 

 in a cloaca (cl) which apparently opens also on the ven- 

 tral side. In all other rotifers without exception having 

 a cloaca this organ opens on the dorsal side. Surgeon 

 Gunson Thorpe was so impressed by this anomaly in his 

 new SDecies that, rather than admit that this was a new 



ST J 



departure, he preferred to declare that the side on which 

 both the mouth and cloaca open is really the dorsal side 

 of the animal. But then the usual dorsal gap in the cili- 

 ary wreath and single dorsal antenna (d. a.), which are 

 both very distinct, would in this animal be situated on the 

 ventral side. However, there is a better way out of the 

 difficulty, the cloaca is only apparantly ventral when in 

 reality it is dorsal. In the rotifers having no foot there 

 is no clear indication as to where the dorsal side begins ; 

 the best explanation in this case is to say that the ven- 



