Figure of Rotifer, 241 



is described as such in V. Cams' Icones Zootomicae, Taf, viii., 

 • fig-. 16, in Prostonmm lineare, is really homologous with the 

 proboscis of the Nemertines^ and probably also with a 

 rudimentary structure which by careful focussing can be 

 seen in the central lobe of the three into which the frontlet 

 of many Dendrocoela is divided, as in this specimen. The 

 Turbellarians never possess suckersj differing herein from the 

 two other orders of Platyelminthes, the Trematodes and the 

 Cestodes; and the structure which has been described in 

 Prostonmm lineare as being a sucker, has been shown by 

 Claparede to communicate with its comparatively simple and 

 aproctous digestive sac. See Claparede, Beobachtungen iiber 

 Anatomic und Entwickelungsgeschichte Wirbellos. Thiere, 

 1863, pp. 16, 17, Taf. iii., fig. 3. 

 Anterior coecal end of intestine, passing upwards between the 

 two eyes, and seen very clearly in the marine sj)ecies allied to 

 this one to be underlaid by a band of nervous tissue, passing 

 across as a commissure between the two nerve-ganglia in 

 relation with the eyes. Of the presence however of even 

 the nerve-ganglia it is not always easy to convince oneself 

 with the semi-transparent fresh-water species Dendrocoelum 

 lacteum under the microscope, and it must be borne in mind 

 that in comparative anatomy, as in development, the evolu- 

 tion of the organs of special sense may take precedence of 

 the differentiation of central nerve-organs (see p. 157, supra). 

 The digestive tract between the opening into it of the 

 pharynx and this anterior coecal end gives off from eight to 

 eleven lateral branches ; two other branches pass back from 

 the point of junction of the digestive tract with the pharynx, 

 and surround the area in which that organ, as also the 

 orifice of the bisexual generative glands, and the male in- 

 tromittent and another organ of uncertain function, are 

 seen to be situated. These two branches fuse posteriorly, 

 and from the arch formed by their anastomosis numerous 

 branches pass backwards and outwards. This arborescent 

 form of intestine is always correlated with the absence 

 of an anus in the Turbellarians ; and in the Rhabdocoelous 

 Prostomum and Vortex, where the digestive tract is aproc- 

 tous, indications of a tendency to form lateral diverticula, 



