NERVOUS SYSTEM AND SENSORY ORGANS. 



349 



KiNGSLEY. The ventral chain of ganglia thus develops essentially in 

 the manner described above as typical for the Crustacea (p. 160), and 

 apparently for all Arthropoda. There are, in all, eight distinct pairs 

 of ventral ganglia in the embryo, the six anterior pairs occurring on 

 the six thoracic segments. As the oral aperture shifts back, the 

 anterior pair of post-oral ganglia, those belonging to the chelicerae, 

 moves gradually forward, so that finally these ganglia become incor- 

 porated with the oesophageal commissures proper, the nerves whieli 

 run to them originating close to the edge of the brain. The thoracic 

 portion of the ventral chain of ganglia is at first elongated. 

 Only in later stages does it become concentrated anteriorly, the 

 circular form characteristic of the adult arising at the same time 

 1)y the shifting apart of its two halves. Nothing is as yet 

 accurately known as to the origin of the vascular sheaths of 

 this part of the 

 nervous system 

 in Limulus, but 

 KiNGSLEY (No. 15) 

 states that they 

 pass through a 

 staire in their de- 

 velopment which, 

 in the Scorpiones, 

 persists through- 

 out life, where 

 they are found 

 lying upon the 

 oesophageal com- 

 missures Avithout 

 completely envel- 

 oping them. 



As to the development of the siqn-a-oesophagcal ganglion proper, or brain, 

 which gives off the optic and some integumental nerves (the frontal nerves), 

 we have only a few short statements by Patten (Nos. 28 and 29) and Kingsley 

 (No. 15), which throw but little light on the actual facts concerning the com- 

 plicated processes which here appear. According to Patten, with whom 

 Kingsley agrees, the rudiment of the brain consists of three consecutive pairs 

 of ganglia, which represent a pre-oral continuation of the ventral chain of ganglia 

 lying in the cephalic region of the body. (This is in agreement with the scheme 

 drawn up by Patten for the Scorpiones and Aci/ius, and described in the 

 section on the nervous system of the Insecta, Vol. iii. ) Each of these three 

 pairs corresponds to a pair of ectodermal invaginations which original!}' lie on the 

 outer sides of the ventral chain, and give rise to the optic ganglia. In Limulus, 

 the two anterior pairs of invaginations are said to unite to form the median 



a. 



Fig. 102.— Diagram of the germ-disc of Limxdns with the lateral, 

 segmental sensory organs (after Patten), a, anus ; to, mouth ; 

 '/■, margin of the cephalo-thoracic shield ; s, sensory organ of 

 the third cerebral segment, so^-so^^ first six lateral, segmental 

 sensory organs; so^, dorsal organ, 1-G, the six pairs of thoracic 

 limbs. 



