THE PROBLEM OF THE PYCNOGONS 689 



According to one view the ovigers of the Pycnogonida are 

 homologous with the first pair of legs in the Arachnida, while 

 the last pair of legs in the former group represent the first of the 

 abdominal (mesosomatic) appendages of Arachnida, or, accord- 

 ing to Lankester and Pocock, those of the pregenital somite 

 which is without appendages in all existing Arachnida. On this 

 view the coincidence in the number of the walking-legs in the 

 two groups loses the significance attributed to it by the older 

 observers. A more serious objection is that the distinction 

 between prosomatic and mesosomatic appendages is sharply 

 marked in all Arachnida known to us from Silurian times down 

 to the present. It may be laid down as a general rule that when, 

 in any division of the Arthropoda, a grouping of the appendages 

 into distinct series or " tagmata" becomes well established, it is 

 rarely, if ever, completely obliterated in the further evolution of- 

 the group. For example, among Crustacea, the grouping of the 

 trunk-limbs into the two tagmata of the thorax and abdomen, 

 which is one of the most distinctive features of the sub-class 

 Malacostraca, never becomes lost amid the varied modifications 

 which these limbs undergo. Accordingly, the absolute identity 

 in structure between the last and the preceding pairs of legs 

 in Pycnogonida would seem to imply, on this view of their 

 homologies, that the delimitation of the prosomatic and meso- 

 somatic regions had not been established when the Pycnogonida 

 diverged from the main Arachnid stock, which takes us a very 

 long way back indeed. 



Another view of the homologies of the Pycnogonid limbs is 

 that advocated by Carpenter, and is based on an observation 

 by Lendl, who states that, in the development of a spider, a 

 pair of transitory rudiments of appendages make their appear- 

 ance between the chelicerse and pedipalpi. There is thus 

 provided an equivalent for the palpi of the Pycnogonida, and 

 we are permitted to retain the obvious and traditional com- 

 parison between the eight legs of the " Sea-Spiders " and those 

 of the more normal Arachnida. The only objection to accepting 

 this as a satisfactory solution of the problem is that Lendl's 

 observation stands, as yet, quite alone. His account is, indeed, 

 clear and detailed, but it is not accompanied by figures, and 

 none of the other investigators of Arachnid embryology appears 

 to have seen the rudiments which he describes. It is much to 

 be desired that the subject should be reinvestigated. 



