cxxxviii Introduction. 



Class, Rotifer a. 



Vermes with a retractile ciliated disk at the anterior extremity of 

 their bodies, which are ordinarily microscopic in size, though they 

 may attain as large a size as -gV^li of ^^ i^^ch in length. They are 

 usually more or less plainly annulated externally, but they never 

 are divided internally into compartments by any transverse septa. 

 In most Rotifera the entire body is divisible into a ' body ' proper, 

 and a tail or foot, anteriorly to which the digestive and reproductive 

 viscera with their ducts and outlets ai*e situated. The ' body ' can 

 often be seen, when chitinization has not advanced so far as to form 

 a carapace^ not only to be distinctly annulated, but to possess both 

 circular and longitudinal muscles in its walls. Cilia are never found 

 on the external surface of the body, except upon the cephalic organ, 

 whence they take their name ; the chitinous surface of the integu- 

 ment may develope setiform outgrowths of various shapes, or the 

 animals may secrete or agglutinate a tube for the lodgment of their 

 body, or may clothe themselves, as Notommata centrura, with a 

 mucous envelope. The ' tail ' is usually annulated when its integu- 

 ment is soft, or segmented when it is indurated ; it often carries 

 paired claw-like processes, or a suctorial disk terminally, and it may 

 be ciliated externally. Internally it contains muscles and a peculiar 

 glandular body. It has been considered to represent a fused pair of 

 arthropodal appendages, by those naturalists who would class the 

 E-otifera with Arthropoda ; it is more correct to compare it with 

 that portion of the body of a free Nematoid, anteriorly to which 

 the anus and generative ducts open ; and though the anal outlet 

 is upon the ventral surface in the Nematoids, and upon the dorsal 

 surface anteriorly to the foot in the Rotifers, it must be recollected 

 the anus is placed dorsally in a very great number of Vermes, as 

 well as in a few of the lower Crustacea with which the Rotifera 

 have been supposed to be allied. The opening, however, of the 

 genital organ on the dorsal surface, is a point in which the Rotifera 

 stand alone among Vermes, but in which they resemble not only 

 the Arthropoda in question, but also many Echinodermata. 



The muscles of the Rotifera appear to be, in some cases at least, 

 transversely striated ; but their ciliated ' rotary ' disk is in most the 



