42 J. F. MCCLENDON ; 



pairs of " haemal " nerves, and each neural nerve had a ganglion at 

 its base. The first neuromere was nerveless ; the second had a pair 

 of neural nerves to the median eyes and a pair of haemal nerves 

 to muscles ; the third neuromere had a pair of neural nerves to 

 the lateral eyes and two pairs of haemal nerves to muscles ; neur- 

 omeres four to nine had each a pair of neural nerves to append- 

 ages and two pairs of haemal nerves to internal muscles. The 

 last four neuromeres had their neural nerves all united into one 

 pair, that innervated the pectines, but each had two pairs of hsemal 

 nerves which went to make up a system homologous to the vagus 

 nerve of vertebrates. In each of the neuromeres succeeding the 

 "brain " there was one pair of neural nerves, and only one pair 

 of haemal nerves well developed. These two pairs of nerves 

 united a short distance from the nerve center to form a single 

 pair homologous to spinal nerves of vertebrates. Patten figured 

 the pits that form in the neuromeres in the early embryonic stages, 

 and he supposed that they represented sense-organs from which 

 the nervous system arose. In the course of development a piece 

 is constricted off of each ganglion of the ventral chain in each 

 segment and added to the succeeding ganglion. 



Viallanes' work (1893, 2) was chiefly on Limulus and does not 

 bear directly on our subject, but it is interesting to note that he 

 held the view that there is only one pre-cheliceral neuromere in 

 the arachnid type of nervous system. 



Braner (1894-5) worked out the earlier half of the embry- 

 ology of the scorpion. He clearly described and figured the 

 early development of the eyes and their relation to the nervous 

 system, a matter that had been confused by all previous workers 

 on the subject. He supported Kowalevsky and Schulgin's ex- 

 planation of the pits in the embryonic nerve tissue, rather than 

 Patten's. The ectoderm was found to grow over the ventral 

 chain from the sides. Brauer reckoned the number of neu- 

 romeres by the cross-commissures, there being two in front 

 of the cheliceral segment. But it is not evident why he 

 should count cross-commissures rather than nerves or ganglia, 

 which are equally characteristic parts of a neuromere. Such 

 a criterion for neuromere would hardly be accepted in a form 

 like Pcripatus. 



