428 Causes and Course of Organic Evolution 



organs of even more elaborate structure open at the anterior 

 or antero-lateral part of tlie head, and He in front of, or lateral 

 to, the cerebral mass. As described and figured by Von Graff, 

 these seem to be paired olfactory organs, of more elaborate 

 detail than in the former type. 



Eyes occur in the majority of the rhabdocoels, though not 

 a few species want them. Usually two, sometimes four, occa- 

 sionally more true pigment eyes are developed. In position 

 they may be lateral and paired, or two may remain so, while 

 the other pair may fuse in the middle line, or one of the two 

 in a pair may be placed in front of the other. Such relations 

 exactly recall conditions seen in some nemerteans, and explain 

 the commencing origin of the paired and the median eyes of 

 cyclostomes and higher vertebrates. It would be impossible 

 here to deal ^\'ith the refractive non-pigmented organs described 

 by Von Graff, though their possible phylogenetic relation is 

 most suggestive. 



The statocyst or probable geotactic organ is slightly ad- 

 vanced on that of Rotifera, but in the size of the statocvst 

 and statolith, as well as in the nerve-supply, it is intermediate 

 between those of Rotifera and Nemertinea. 



In a few of the genera — and these mainly advanced in struc- 

 ture and marine in habit — two lateral ciliate furrows appear 

 below and behind the brain, and by their upper ends extend 

 at times near to the statocyst. They are ciliate cavities of 

 greater or less depth and extent. The opinion might be ven- 

 tured that these represent the common beginnings of a first or 

 branchial furrow and auditory sac, such as in the nemerteans 

 become the "head furrows." Reference is made to these later. 



The frontal organs of nemerteans consist either of one (Eu- 

 polia) or three (Cerebratulus) slightly elevated but at times 

 retractile papillae into whose apex open the ducts of gland 

 cells. Sensory nerves also pass directly to them from the 

 brain. They have been variously regarded as tactile organs 

 or taste papillae. From their position, nerve supply, and glandu- 

 lar supi>ly they seem almost certainly to represent a rudi- 

 mentary olfactory system, a view further strengthened by 

 their becoming retracted or depiessed into a slight pit. As 

 described and figured by Burger also, the epithelial cells of 

 each depression consist of ciliated sensory or olfactory cells, 

 that alternate "^Hith interstitial or supporting cells, an arrange- 

 ment exactly similar to that seen in the vertebrate olfactory 

 organ. 



In cyclostomes alone, amongst vertebrates, the nostril and 

 olfactory organ are single as in metanemerteans, but. since 



