34 T. H. MORGAN, 



acquired in a much later stage and transmitted to larval life." 

 That is to say, that almost every characteristic of the larva 

 has been handed back from the adult! And what remains? 

 " Nothing but the nervous system ... an intestine, three pairs 

 of appendages and two eyes." " But these are the attributes 

 which one finds equally in the larva of Annelids !" 



There is one vital fact which has been left out of account, viz. 

 the presence of an anus in the Trochophore. How can we account 

 for its absence on the above hypothesis when we know the 

 Pantopod-larva to be a free-living larval form? And unless 

 some special reason for such a loss can be imagined, the very 

 basis of the comparison between Pantopod-larva and Trochophore 

 is gone! What we have done in the above process of subtrac- 

 tion is to have removed the most striking structures of the adult 

 from the larva and have left, not a Trochophore, but only the 

 framework of the Pycnogonid. 



For two main reasons I am unable to believe in the phylogeny 

 given by Dohrn or by Hoek. First, because it seems to me there 

 are facts derived from the early stages of development which 

 point unmistakably to a relationship between Pycnogonids and 

 Arachnids; and in the second place I cannot believe any actual 

 homology to exist between the Fantopod-larva and the Trocho- 

 phore, nor any fair reasons to assume that the characteristics of 

 the Fycnogonids have been reflected upon a Trochophore. 



If, then, we start with the assumption that there is a relationship 

 between the Sea-Spiders and Arachnids, we may examine into 

 the meaning of the larval form as a corollary to such a position. 



I have stated that the Fycnogonids are degenerate, and prob- 

 ably not derivable directly from any existing group. If so, how 

 far down in the ancestral tree of the Arachnids have they arisen ? 

 The very great differences in the adult structure of the groups 

 indicate no very recent origin, but possibly they came in at a 

 time when the Arachnids had the first pair of appendages chelate, 

 and these were innervated from the supra-cesophageal ganglia, and 

 had coeca from the digestive tract into most or all of the append- 

 ages. After the divergence of the Pycnogonids as a group from 

 the general phylum of the Arachnids, the Pantopod-larva may 

 have developed. 



The Pycnogonids have adapted themselves to a very special 



