688 SCIENCE PROGRESS 



of the Crustacea. The typical members of that class have 

 two pairs of appendages in front of the mouth (antennules and 

 antennae), and at least three pairs of post-oral appendages of 

 which the basal segments act as jaws (mandibles, maxillulae 

 and maxillae). The details of the appendages and the structure 

 of the internal organs give no evidence in favour of an alliance 

 between the Pycnogonida and the Crustacea. A peculiar larval 

 stage, common to many Pycnogons, in which only three pairs of 

 appendages are present, has been compared with the nauplius- 

 larva of Crustacea, but the resemblance hardly extends beyond 

 the agreement in the number of the limbs. The chelate form 

 of the first pair and the uniramous character of the second and 

 third, as well as the presence of a suctorial proboscis, are 

 important points of divergence from the nauplius type. 



The general resemblance to an Arachnid which led Strom, 

 in 1762, to give the name Phalangium marinum to the first 

 described Pycnogon, is due chiefly to the four pairs of long and 

 slender legs. It is increased by the chelate form of the single 

 pair of appendages which lie in front of (or above) the mouth, 

 and correspond to the chelicerae of the Arachnida. Beyond this, 

 however, the comparison of the appendages becomes involved in 

 difficulties, for the Arachnids possess but one pair of appendages 

 (the pedipalps) between the chelicerae and the first pair of legs, 

 while in the Pycnogons two pairs, the palps and the ovigers, 

 occupy the same position. 



Some zoologists who appear to estimate the relative value of 

 facts according to the difficulties in the way of their investiga- 

 tion, attach greater importance to the internal anatomy of the 

 Arthropoda than to their external characters, and to the remoter 

 details of their embryology than to the general plan of structure 

 of the adult animals. While evidence from all these sources 

 must be allowed due weight in attempting to decide as to 

 the systematic affinities of organisms, it seems clear that, in the 

 present case, parallels between the Pycnogonida and Arachnida 

 in the hepatic diverticula of the alimentary canal and in the 

 mode of formation of the germ-layers in the embryo can hardly 

 be brought into the argument while we are faced by a serious 

 discrepancy in the comparison of the somites and appendages of 

 the two groups. 



Attempts to minimise or to explain this discrepancy have not 

 been wanting, but only two of these can be mentioned here. 



