NBMERTEA ENOPLA 641 



Eeptantia the dorsal lobe attains its greatest development in 

 its hinder part, when the ventral lobe has disappeared. A 

 section of Drepanophorus albolineatus (Text-fig. 14, b) 

 to be compared with Text-fig. 14, a, reveals the difference 

 between the two. The proportions have changed ; instead of 

 the ventral the dorsal ganglion exceeds in Drepanophorus, 

 and if we compare a section of another form, in which the 

 nerve-cord is seen instead of the ventral lobe, the contrast is 

 still more obvious (Text-fig. 14, c). I cannot agree with 

 Brinkmann that the disappearance of the cerebral organs 

 caused a reduction of the dorsal brain-lobe in Pelagica. 

 Brinkmann considers these to be descendants of true 

 Drepanophoridae that have lost eyes, cerebral organs, the great 

 development of the brain, the nephridia, the rhynchocoelomic 

 diverticula, the anastomosing blood-vessels. As to the eyes 

 I might agree with him ; it seems quite plausible that species 

 or even genera that live in the aphotic regions of the sea lose 

 their eyes, as most Pelagica, Siboganemertes, and Uniporus 

 hyalinus, though for Uniporus acutocaudatus and 

 U. borea lis this reason cannot exist as they live in the dyspho- 

 tic zone just as well. The presence of atrophied eyes in some 

 pelagic genera, however, makes it probable that they got lost 

 in the other. But certainly the Pelagica never possessed cere- ] 



bral organs. These have developed in different ways in armed I 



and unarmed Nemerteans. In both sub-classes we know - 



genera without them, and these in both are primitive forms. 

 Callinera, Carinesta, Cephalothrix belong to the most primitive 

 Palaeonemerteans and they have no cerebral organs. In the 

 Enopla this sense organ is absent in the Pelagica and in Mala- ^""""^v r ^^ 

 cobdella. The parasitic genus Gononemertes has them and in ^i\^9^^\vP»^ 

 Carcinonemertes they seem to fail. Wliy must it have got ^^ I 



lost in Malacolbdella and Carcinonemertes, when it is present ^ 



in the third parasitic genus ? As to Carcinonemertes, that I 



belongs to a non-parasitic family of Monostilifera with well- 

 developed cerebral organs, it seems natural to consider the , 

 parasitic habits of the genus as the cause of their absence, i 

 though nothing is less certain. In Bdellonemertea this 



NO. 268 u u I 



