ARACHNIDA. 637 



lobes have developed, and the blastoderm has divided into a 

 dermal, or outer layer, and a muscular, or inner layer of 

 cells. The outer layer forms the chitinous body-wall, or crust, 

 while from the inner layer are developed the digestive, vascu- 

 lar and other organs besides the muscles. 



After the rudiments of the appendages are formed the epi- 

 mera appear. At this period we are struck with the perfect 

 identity between all the appendages of the body at their first 

 origin. In the Arachnida the formation of the primitive seg- 

 ments takes place much sooner than in most other articulates, 

 where they often do not appear until after the rudiments of the 

 limbs are developed. 



Another characteristic of the evolution of the spiders is 

 the tardy appearance of the rudiments of the .legs. The ven- 

 tral arcs, or protozoonites, subdivide into ventral and pleural 

 parts, which signalize the formation of the permanent rings 

 of the body. The author's figures 

 and statement show, though he 

 does not state the fact clearly, 

 that development progresses from 

 each end of the body towards the 

 centre, as we have shown* to be 

 the case in insects. Thus the 

 posterior half of the body repeats 

 the mode of development and 

 general form of the anterior, or 

 cephalic pole. 



The third period in the life of 

 the embryo dates from the forma- Fig. 626. 



tion of the ventral rudiments to the exclusion of the spider. 

 The first change consists in the lengthening and meeting 

 of the rudimentary legs. The mouth-parts develop first. 

 At this period the limb-bearing (pleural) region of the 

 body separates and the sternal piece or breast-plate appears 

 as a "slower, later formation." Now the thoracic legs grow 

 much more rapidly than the mouth-parts and lie interlocked 

 upon the breast. (Fig. 626. f) When the first pair of legs are 



* Proceedings Boston Society of Natural History, Feb. 7, 1866. 

 . 626, m, mandibles; mx, maxillae; I, fourth pair of legs; p, postabdomen. 



