ORDER GASTROTRICHA. 195 



backwards in the manner of a paddle and so serve as locomotor organs, pro- 

 ducing a quick jerky movement quite different from the steady progression 

 caused by the cilia of the trochal disk. In another nearly-related form, 

 Pedalion, six processes are also present, but are arranged somewhat differ- 

 ently from those of Hexarthra, the largest one arising from the ventral 

 and another from the dorsal surface, while the other four are lateral in 

 position, two occurring on each side. 



The Affinities of the Rotifer a. — Several views have been advanced as 

 to the afflnities of the Rotifers, especially as regards their relationships to 

 higher forms ; these opinions will not, however, be fully considered here, 

 but merely indicated, attention being directed first to the relationships in 

 which the Rotifers stand to organisms lower in the scale. In this connec- 

 tion the excretory system becomes of no little importance on account of its 

 resemblance to that of the Turbellaria, a resemblance which is further 

 emphasized by the nervous system, — consisting of the simple brain, from 

 which posteriorly-directed nerve-cords arise, — by the combined ovary and 

 vitellarium, and by the absence of a blood vascular system. Here, how- 

 ever, the resemblance ceases, and the presence of an anal opening to the 

 digestive tube marks the Rotifers as standing on a higher level than the 

 Turbellaria. It seems probable, however, that the similarities do indicate 

 the ancestry, and that the Rotifera have been derived from the Turbellarian 

 type. 



Another possibility which has been suggested is to the effect that they 

 are derived from the form represented by the Trochophore larva of the 

 Annelida (see p. 313). The principal argument for this view is found in the 

 arrangement of the trochal cilia, which, in the occurrence in many cases of 

 both prsBoral and postoral bands, certainly resembles not a little that of 

 the Trochophore larva. It must be remembered, however, that the similar- 

 ity in the arrangement of the cilia is not quite perfect, and that it may be 

 without phylogenetio significance, having been acquired independently in 

 the Rotifers and in the Trochophore larva ; and furthermore it is noticeable 

 that in one important character at least a marked difference is found, the 

 nervous ganglion lying in the Rotifers behind instead of before the prseoral 

 band of cilia. The most that can be said at present is that the Rotifers 

 show closer structural afiinities to the Turbellaria than to any other group, 

 and that it is probable that they represent the culmination of a line of 

 development originating in that group, and furthermore that it is possible 

 that they represent the ancestral annelid form indicated by the Trocho- 

 phore larva. 



Order Gastrotricha. 



The Gastrotricha are minute forms, few exceeding 0.2 mm. 

 in length, which occur in fresh water and have an elongated 

 form flattened somewhat on the ventral surface, tapering 



