338 



ARTHROPOD A. 



The supraoesophageal ganglia are formed as two independent 



thickenings of the procephalic 

 lobes (tig. 244), which event- 

 ually separate from the su- 

 perficial skin. There is formed 

 however in each of them a 

 semicirculargroove (fig. 244, yr) 

 lined by the superficial epi- 

 blast, which becomes detached 

 from the skin, and is involuted 

 to form part of the ganglia. 



A similar mode of forma- 

 tion of both the ventral cords 

 and the supraoesophageal gan- 

 glia obtains in Insects (fig. 

 24-5). The ventral cords are 

 however much less widely 

 separated than in Spiders, and 



FIG. '24'2. HEAD OF AX EMBKYO PEHIPATUS. 

 (From Moseley.) 



The figure shews the jaws (mandibles), and 

 close to them epiblastic involutions, which 

 grow into the snpraoasophageal ganglia. The 



antennre, oral cavity, and oral papillae are also 

 shewn. 



early unite in the median line. 

 In the supraoesophageal gan- 

 glia the invaginated epiblast 



has in Lepidoptera (Hatschek) the form of a pit on the dorsal border 



of the antennae. 



Hatschek states that there takes place an invaginatioii of a median 

 part of the skin between the twu ventral cords, for the details of which I 

 must refer the reader to Vol. I. p. 340. He has made more or less similar 

 statements for the earthworm, but his observations in both instances are 

 open to serious doubt. 



r 



. : ; ^ir^ 

 



FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA LABYRINTHICA. 



The ventral cords have begun to be formed as thickenings of the epiblast, and the 

 limbs are established. 



me.s. mesoblastic somite ; nt. ventral nerve-cord; yk. yolk. 



Full details as to the development of the nervous system in the 

 Crustacea are still wanting ; a fairly complete account of what is 

 known on the subject is given in Vol. I. pp. 433 4. It appears that 

 the ventral cord may either arise as an unpaired thickening of the 

 epiblast (Isopoda), marked however by a shallow median furrow, or 



