156 MARINE BIOLOGICAL LABORATORY. 



spiders with the first of the sea-spiders, the third with 

 the second, and the fourth with the third, leaving over 

 the fourth pair of legs of the Pycnogonids as a pair not 

 found in the spiders. If this be true the position to 

 which we are forced is obvious, — that the last segment 

 of the body of the sea-spiders, and its appendages, corre- 

 spond to the first segment of the abdomen of the spiders. 

 Such a transfer of segments is not uncommon amongst 

 Arthropods, and there is nothing unusual in such a 

 process. It here carries with it, however, the assump- 

 tion that the fusion must have taken place at a time 

 when the abdomen still bore appendages serially homolo- 

 gous with those of the thorax. We may imagine, if we 

 like, that this took place at a time when the third pair 

 of appendages appeared and began to carry the eggs, 

 so that the body, by utilizing the first legs as egg- 

 carriers, retained a pair of the abdominal legs for pur- 

 poses of locomotion. 



I must confess, however, that although this last view 

 seems far the more probable, in the present state of our 

 knowledge, yet the idea is a very attractive one, — that 

 the four pairs of walking-legs are homologous in the 

 two groups, and that therefore the spiders have lost a 

 pair of appendages between the chelae and the first pair 

 of legs. 



I have, in the above comparison, left to one side the 

 Crustacea, nor is there need to say much in regard to 

 their possible relationship to the Pycnogonids. The 

 characteristics which the two groups have in common 

 are only those of Arthropods in general. Other than 

 this there seem to be few homologies, with the possible 

 exception of a six-legged larval form of which I shall 



