i6 



OUTLINE OF THE ARACHNID THEORY. 



In the insects, the first four metameres fuse with each other, and with the 

 procephalon, to form the so-called "head," the last three metameres usually 

 remaining separate. (Figs. ^,A, 5,4.) 



In phyllopods (Branchipus) the first two metameres, and possibly an evanes- 

 cent third, or premandibular, fuse with each other and with the procephalon. 

 The remaining three metameres, the mandibular and two maxillary, fuse with 

 each other, forming a group by themselves distinct from the anterior division. 



In arachnids, such as the scorpions, spiders, trilobites, and merostomes, 

 all six thoracic metameres unite with one another and with the procephalon to 

 form the cephalothorax, leaving on the hsmal side little or no indication of the 

 larger divisions, or of the more primitive division into metameres. 



On the neural side, the metameric structure is 

 always retained. The reduction in size, and the 

 modification of the first two or three pairs of appen- 

 dages for feeding purposes, are usually clearly indica- 

 ted, the last three or four pairs serving mainly for 

 locomotion. 



In Limulus, the subdivision of the thorax into 

 an anterior and a posterior division at first sight does 

 not appear to exist; but I have shown that in abnor- 

 mal embryos the first two or three thoracic metameres 

 act as a unit, in that they frequently separate from 

 the posterior ones opposite the large thoracic sense 

 organs, or they fuse with each other, or disappear 

 entirely, or otherwise manifest a distinct independence 

 in their development. The large thoracic placodes, 

 the forerunners of the auditory placodes of verte- 

 brates, mark this latent cleavage line between the 

 ■Mesothyra (after Hall and group of Oral mctamercs of the diaccphaloH, and those 



:). Upper Devonian. i i • j_ j.i i i /-i-«' 



belonging to the mesocephalon. (Figs. 141, 142, 

 184-188.) 



The endocranium arose primarily in association with the dicephalic met- 

 ameres, but in the higher forms takes its origin from the mesocephalic meta- 

 meres also. With the concentration of all the cranial neuromeres, the endo- 

 cranium embraces, or underlies all of them except the more posterior ones of 

 the branchiocephalon. 



Fig. 10 

 Clark) 



Oral Arches. — The basal joints of the thoracic appendages, especially in 

 the arachnids, are greatly expanded where they join the body, forming oblong 

 arches to which the slender, more movable part of the appendage is attached. In 

 the arachnids, these basal arches may be located some distance from the median 

 line, on the lateral wall of the head. At least four or five of these anterior thoracic 



