AGELENA N^VIA WALCKEN^R 161 



number of somites which enter into the formation of the arachnid 

 body, is still divided. Some maintain that the portion of the 

 head situated in front of the mouth is composed of two somites 

 only. In this case the chelicera would be homologous with the 

 antennules of the lobster. A more generally accepted opinion 

 is based on the assumption that the chelicera are homologous 

 with the antennas of the lobster, and that the preoral region is 

 composed of three somites. We, for our part, will assume that 

 four somites lie in front of the mouth and that the entire body of 

 an arachnid is composed of twenty- two somites, i. e., as many as 

 in the crustacean Nebalia and one more than in the lobster. 

 According to this interpretation the chelicera are homologous 

 with the mandibles of insects and crustaceans. They are postoral 

 in the embryo, but become preoral early in the development. 

 The twenty- two somites are distributed as follows: five belong 

 to the head, four to the thorax, and thirteen to the abdomen. 

 The four preoral somites of the head are (i) the first ocular 

 somite, characterized by the presence of the four lateral and 

 two posterior median eyes; (2) the second ocular somite, char- 

 acterized by the presence of the two anterior median eyes; (3) an 

 embryonic, evanescent segment with rudimentary appendages 

 which disappear completely; (4) the cheliceral somite with 

 chelicera for appendages. The fifth cephalic somite is the first 

 postoral somite. It has the pedipalpi for appendages. The four 

 thoracic somites have a pair of legs each. The abdomen in the 

 embryo of Agelena is divided into ten segments, the first six 

 of which have a pair of embryonic appendages each. The first 

 to ninth segment represent as many somites, but the tenth seg- 

 ment evidently corresponds to the last four segments of the 

 abdomen in the scorpion and is therefore the result of a fusion 

 of four somites. The embryonic appendages of the first, second, 

 third and sixth abdominal somites disappear, those of the fourth 

 and fifth become spinnerets. The first abdominal somite which 

 disappears completely in the scorpion, forms the pedicel in 

 Agelena. The pair of lungs and the genital opening belong to 



