548 E. KAY LANKESTEK. 



and that the gnatliobases of the cheire which act as the first 

 pair of hemiguaths, are carried by the third somite. The 

 Arachnida are therefore tritoguathous. The two prostho- 

 raeres are indicated by their coelouiic cavities in the embryo 

 (I and II, fig. 4), and by two neuromeres^ the protocerebrum 

 and the deuterocerebrum. The appendages of the first 

 prosthomere are not present as tentacles, as in Peripatus and 

 Diplopods, but are possibly represented by the eyes or possibly 

 altogether aborted. The appendages of the second prostho- 

 mere are the well-kuoAvu cheliceras of the Arachnids, rarely, 



Fig. i. — Diagram of the head and adjacent region of an Arachnid. 

 Diprosthonierous in the adult condition, though cmbrjologically the 

 appendaL:es of somite II and the somite itsell are, as Jiere drawn, 

 not actually in front of the mouth. E, lateral eye ; Ch, chelicera ; 

 m, mouth ; P, piotocerebrum ; I>, deuterocerebrum ; I, II, III, IV, 

 ccelom of the tirst, second, third, and fourth somites. (After 

 Goodrich.) 



if ever, aiitenniform, btit modified as " I'etroverts " or clasp- 

 knife fangs in spiders. 



The Crustacea (fig. 5) and the Hexapoda (fig. 6) agree in 

 having three somites in front of the mouth, and it is probable, 

 though not ascertained, that the Chilopoda (Scolopendra, etc.) 

 are in the same case. The three prosthomeres or pra^oral 

 somites of Crustacea due to the sinking back of the mouth 

 one somite farther than in Arachnida are not clearly indicated 

 by coelomic cavities in the embryo, but their existence is 

 clearly established by the development and position of the 

 appendages and by the neuromeres. 



