442 Causes and Course of Organic Evolution 



of the two lateral eyes of cyclostomes, and equally for the 

 rudimentary pineal and parapineal eyes of this group. In 

 this connection comparison might be made of the figure in Bur- 

 ger's work (PL 10) for two of the multiocellate organs of Drep- 

 anophonis, with that from Studnicka of the pineal and para- 

 pineal eyes of Petromyzon. As regards the region of the brain 

 from which the optic nerves spring, Burger states (p. 171) 

 that in nemerteans they arise from "the anterior circumference 

 of the brain." How far this might be correlated more exactly 

 with the habenular and posterior commissure of the brain 

 in Petromyzon yet remains to be determined. 



But we would trace a direct continuity from the four-eyed 

 state, typical of various land and fresh-water nemerteans, 

 in which two of the eyes form one paired series and two form 

 another unpaired series and often also in reduced state, to 

 the paired eyes and the rudimentary pineal and parapineal 

 eyes of Petromyzon, and from this again to the vestigial pineal 

 eyes of amphibians and mammals. 



While the auditory sac is closed off in its endolymph canal 

 from the exterior in vertebrates generally, it is worth noting 

 that in elasmobranchs an orifice exists, the possible origin 

 of wliich has been already explained (p. 431). The simple 

 ear of Myxine is an advance on w^hat we regard as the nemer- 

 tean ear, in that a semicircular canal has developed along 

 with two sacs. The presence of two semicircular canals in 

 Petromyzon, along with other details that are typical for higher 

 vertebrates, leads us to the ear of the latter, with its three 

 semicircular canals. 



A blood-vascular system seems to be wholly undeveloped 

 in the Rotifera. In the Rhabdocoela, it is also absent as such. 

 But in view of the extensive and varied ramifications of the 

 excretory system, as described and figured by Von Graff {131: 

 2137-2151), in view^ also of what is noted below regarding 

 the joint blood-vascular and excretory systems of nemerteans, 

 it seems not unfair to suggest that, in transition from the 

 Rhabdocoela to the Nemertinea, a i)ortion, and specially cer- 

 tain of the main trunks of the excretory system, in the former, 



