ZOOLOGY AND BOTANY^ MICKOSCOPY, ETC. 63 



lower face of tlie cords for tbeir whole extent ; there are no ganglionic 



masses, and no transverse commissures. 



The brain of Polygordius presents us with a higher stage, inas- 

 much as, though still in contact with the ectoderm, it is more distinctly 

 individualized owing to the possession of a proper sheath ; it is more 

 strongly marked off into several ganglionic masses, each with special 

 functions ; the anterior ganglia innervate the tentacles ; the median 

 one, which is the most important portion of the brain, is in special 

 relation to the epidermis of the cephalic segment, and is connected 

 with the rest of the body by the ventral medulla; the posterior 

 ganglia supply the ciliated pits. 



The brain of Saccocirrus is in very intimate relation to the 

 epidermis ; there is no proper sheath, and the distinctness of the 

 ganglia is less complete than in Polygordius ; on the other hand the 

 nervous cells are very distinct in character from those of the epi- 

 dermis, which is not the case in Polygordius. 



Fraipont thinks that E. Van Beneden has demonstrated that 

 Histriobdella is one of the Archiannelides, and a paper in exemplifica- 

 tion of this is to be published by Fottinger.* Polyophthalmus presents 

 us with an advance in the morphological evolution of the nervous 

 system, for this is separated by muscles from the epidermis, though 

 it is connected with it by nervous branches ; the special ganglia are 

 differentiated, and we have here a true chsetopod brain. 



In addressing himself to the difficult question of the origin of the 

 nervous system of Annelids, the author repeats the doctrine of 

 Kleinenberg that Hydra is provided with neuro-muscular elements, 

 and says that these cells function as sensitive peripheral organs, as 

 ganglionic cells, as sensory and motor nerves, and their internal 

 prolongation as muscles. He then refers to the lately expressed 

 opinion of Sedgwick that the whole of the central nervous system of 

 Triploblastica is the homologue of the buccal nerve-ring of Actiniae, 

 and says that, if that view be correct, the polar j)late of the young 

 larvae of Polygordius represents the anterior part of the buccal ring ; 

 against this it may be objected that the independent development of 

 the dorsal and ventral portions of the central nervous system of Poly- 

 gordius hardly favours the view that the two are the homologues of a 

 single ring. An explanation is given in the suggestion that the tardy 

 appearance of the ventral cord of Polygordius is due to the late 

 development of the trunk. Fraipont thinks that we must consider the 

 enormous development of the peripheral nervous system of the 

 pr^oral lobe not as characteristic of the primitive forms of annelids, 

 but as a secondary arrangement due to the great relative size of the 

 pr^oral lobe as compared with the rest of the body. He thinks it 

 probable that Protodrilus, which has a direct development, will be 

 found to have the brain and ventral cord developed contemporaneously. 



If we limit ourselves to forms in which the two parts of the nervous 

 system appear separately, we may ally it to that of other known 

 forms. In the Actiniae there is an ectodermic nervous plexus, and 

 another, which is independent of it, and which is exclusively related 



* Since published. Arch, de Biol., v. (1884) pp. 435-516 (5 pis.). 



