V. 



Notes on the Development and Structure of 



Arachnids. 



IN a recent contribution to Natural Science (pp. 281, 2) I 

 discussed the correspondence of the appendages of Insects and 

 Arachnids in connection with Cholodkowsky's researches on the 

 embryology of the cockroach. It will be remembered that he main- 

 tains the post-oral origin of insect antennae and their consequent 

 homology with the chelicerae of spiders, scorpions, and their allies. 

 In this he is at variance with most modern writers, who, regarding 

 the antennae of insects as arising from the pre-oral region of the 

 embryonic head, compare the chelicerae of Arachnids with the 

 mandibles of Insects. The former, however, was the prevalent view 

 some years ago, and is restated quite recently in the elaborate 

 Memoir (i) on the Anatomy of Arachnids, by Gaubert, who, on 

 this point, follows his master, Milne-Edwards. 



But Gaubert refers to the other view (that of Balfour), and 

 admits that considerable support has been lately given to it by the 

 researches of Jaworowski (2, 3). The latter observer has studied the 

 embryology of a large " tarentula " or hunting spider {Trochosa 

 signoriensis), and he describes and figures a pair of structures which 

 arise behind the mouth and in front of the chelicerae, which never 

 become jointed, and which ultimately sink into cavities in the 

 blastoderm. These he regards as rudimentary antennae. In front 

 of the segment to which they belong, he describes and figures three 

 other segments, of which only one appears to be pre-oral. By 

 adding these four segments to the six which bear the appendages 

 which persist in the adult spider, we see that the cephalothorax is 

 formed by the fusion of at least ten segments. Though no previous 

 observer has described antennae in an embryo Arachnid, Jaworowski 

 points out the similarity between the rudiments figured by him and 

 a pair of structures, which are shown in exactly the corresponding 

 position in the figure of the embryo oi Agclena labyrinthica in Balfour's 

 " Treatise on Comparative Embryology " (vol. i., fig. 2ood). A 

 comparison between the two figures ' makes it highly probable that 



1 Jaworowski's figures were reproduced in Nature, vol. xlvi., 1892, pp. 272, 3, to 

 illustrate Professor Sollas' review of Simroth's " Die Entstehung der Landtiere." 



