THE EYES. 



241 



In the anterior part of the ganglionic chain a fusion which takes 

 place between several pairs of ganglia leads to the formation of the 

 sub-oesophageal ganglion (Fig. 124, usg), with which a few more 

 may unite. 



As far as we know, no light has as yet been thrown by ontogeny on the subject 

 of the inclusion of the ganglia of true trunk-segments with the brain. The fact 

 that the most posterior section of the brain (the so-called tritocerebrum), forms 

 a part of the circum-oesophageal commissure, and may also possess a special 

 transverse commissure (as in the Diplopoda and Scutigera, according to St. 

 Remy), however, perhaps indicates that the tritocerebrum had an origin similar 

 to that of the maxillary ganglia of Peripatus and the antennary ganglia of the 

 Crustacea, both of which represent trunk-ganglia secondarily united with the 

 brain (p. 193 ; Vol. ii. , p. 164). A diagram of the Myriopodan brain would 

 closely resemble Fig. 97 (p. 193) of the brain in an embryo of Peripatus.* 



The Eyes. 



The formation of the eves has been followed in Julus terrestris, 



a form which possesses a large number of ocelli (about forty) on each 



side of the head. These appear one after the other. The first 



ocellus appears on the fourth day of free larval life, i.e., after the 



Fig. 123.— Section through a developing eye of Julus terrestris (after Heathcote). c/i, chitinous 

 covering of the body ; h, ectoderm beneath the lens (lentigen layer) ; hyp, ectoderm (hypo- 

 denuis) ; k, mesodermal capsule ; I, lens ; /-, retina. 



larva is freed from the cuticular envelope, and is followed gradually 

 by the others until the full number is reached. According to 

 Heathcote, the first step in the formation of the eye is an ectodermal 

 thickening which arises behind the base of the antenna, and in which 

 pigment is deposited. A cavity then forms in the thickened part, 

 so that the whole appears as an optic vesicle. As the cavity increases 



* [In Scolopendra, according to Heymoks (No. III.), the protocerebruin con- 

 sists of (1) the archicerebrum arising in the clypeus, (2) two pairs of ganglia 

 in the primary head-plate, (3) the optic ganglia, (4) a pair of ganglia in the 

 antennular segment. The deutocerebrum is derived from the antennary ganglia, 

 while the tritocerebrum arises in the limbless segment intercalate'! between the 

 antennal and mandibular segments. This shows that at least one pair of trunk- 

 ganglia (two if the antennae are really trunk-appendages) is incorporated in the 

 adult brain.— Ed.] 



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