NERVOUS SYSTEM. 



165 



ontogenetic researches to assume for this region of the body three component 

 parts, homonomous with the other body-segments ; these are the optic, the 

 antemmlar, and the antennal segments. In the optic segment, the optic ganglia 

 would represent the segmental pair of ganglia, while the general position of the 

 parts in the Astacus embryo incline Reichenbach to return to the old view of 

 Milne-Edwards, according to which the eye-stalks represent the limbs of this 

 segment. This last view, which subsequently found supporters in Huxley and 

 Spence Bate,* has, however — and, as it appears to us, rightly — been disproved 

 by Claus and Fr. Muller, by reference 

 to the ontogeny of the stalked eye of the 

 Phyllopodan larva (Branchipus) and of 

 the Zoaea of Lucifer, which indicates 

 that the eye-stalks are to be regarded as 

 secondarily abstricted lateral portions of 

 the head which have become indepen- 

 dently movable, while the optic-ganglion, 

 as a part of the brain shifted anteriorly, 

 also attains only a secondary independ- 

 ence. These conclusions remove all 

 ground for assuming the presence of an 

 independent optic segment. 



With regard to the segment of the 

 body corresponding to the second pair of 

 antennae, there can hardly be any doubt 

 that we here have to do with an origin- 

 ally post-oral body-segment, i.e., with a 

 true trunk-segment, which only second- 

 arily underwent displacement forwards, 

 and thus attained a closer union with the 

 pre-oral parts of the head. This view is 

 supported by the changes brought about 

 during embryonic development in the 

 relative positions of the mouth and the 

 second antennae (p. 157), and above all 

 by the condition ot the nervous system, 

 in which are to be found all transitions 

 between the independent development of 

 the pair of ganglia on this segment and 

 their close fusion with the cephalic mass. 

 It has been known, since the observations 

 of Zaddaoh, that, in Apus, the origin of 



the pairs of antennal nerves is to be sought post-orally in the oesophageal com- 

 missures, and later researches (Pelseneer, No. 14) showed the ganglia at this 

 point to be connected by means of a post-oral transverse commissure, although 

 this has to some extent been otherwise explained. Similar conditions are found 

 in other Phyllopoda. Thus, in Claus' drawing of the brain of a Cladoceran 

 (Fig. 84), three sections can be distinguished, only the two anterior being 

 pre-oral in position. The most anterior section (c 1 , corresponding to Packard's 



* [See also C. Herbst, Uber d. Regeneration v. antennenahnlichen Organen 

 an Stelle v. Augen, Archiv. f Entwickl. Mechanik, Bd. ii., 1895-96, p. 543 ; 

 and B. Hofer, Ein Krebs mit einer Extremitat statt eines Stielauges, Verh. 

 DeuU Zool. GcselL, 1894, p. 82.— Ed.] 



tut* 



Fig. 84. — Ventral aspect of the brain of 

 Daphnia similis(after Claus). c 1 , anterior, 

 &, middle, c3, posterior section of the 

 brain ; go, optic ganglion ; n, nerve of the 

 sensory organ of the neck ; na 1 , nerve of 

 the first antenna; na", nerve of the 

 second antenna ; n', second nerve of the 

 second antenna ; sc, oesophageal com- 

 missure. 



