638 ARACHNIDA. 



long enough to cross each other the jointed structure of the 

 limbs disappears, and they soon become divided into their 

 usual number of joints, though the tarsal joints are the last to 

 be perfected. At this time the maxillae become differentiated, 

 or split up, into the basal lobe and its appendage, or palpus. 

 Claparede compares the basal lobe to the coxa of the legs, 

 though it is formed long before the coxae of the feet them- 

 selves. The anterior pair of appendages form the mandibles. 



The formation of the head is next in order. The "cephalic 

 lobe" is divided into what the author calls two "procephalic 

 lobes," separated by a deep incision, and at this period the 

 head appears very distinct from the thorax. Afterwards the 

 anterior or ante-oral part of the head is, as in the case of the 

 "post-abdomen," folded back on the top, and then closely sol- 

 dered to the thorax, thus forming the so called "cephalo- 

 thorax." These procephalic lobes are separated b}' a third 

 lobe or "triangular plate" which grows up between them, 

 forming the epicldle. The mouth first appears as a longitu- 

 dinal furrow in this triangle, the posterior border of which 

 becomes the so called labium ("glossoide" of Latreille). The 

 labium tlius originates in the spiders in an entirely different 

 way from the appendages, and is not formed, as Brulle sujo- 

 posed, by the soldering of the maxilli«, hence w^e shall adopt 

 Latreille's term "glossoide" for this piece. 



The two procephalic lobes afterwards unite, and are soldered 

 together on the median line, to form the anterior face of the 

 head. Tliis approach takes place from above, over the buccal 

 frame (epichile). The mandibles are thus in advance of the 

 mouth, though primitively behind it. "The head is then in 

 the embryo of the spider very distinct from the thorax. Only 

 towards the end of embryonic life does the soldering of the 

 ' cranium ' and of the prothorax become so intimate that their 

 limits become indistinct. It is only from this moment that 

 there exists a true cephalothorax." (Claparede.) 



Towards the end of embr^'onic life the simple e^es appear, 

 arising from four little furrows, called the "ophthalmic fur- 

 rows." They are colored by the deposition of a small quantity 

 of pigment. The}' appear at an earlier period in the Acarina. 



Formation of the heart and viscera. After the walls of the 



