The Phylogenij of the Pycnogonids. 



It will be impossible to give liere the bearing of these embryological facts 

 upon the phylogeny of the group, and I reserve for the future a fuller 

 discussion. 



It seems to^aie, however, that when all the embryological phenomena are 

 taken together they give quite strong evidence for the relationship of the 

 Pycnogonids to the Arachnids. Dohrn and Hoek have each recently 

 reached independently the belief that the group must be considered an 

 isolated one, with a more or less independent origin from the Annelids. I 

 hesitate before offering an opinion against those who are so well qualified to 

 speak authoritatively on the subject. On the other hand their opinions are 

 based largely on the adult anatomy of the group, as little or nothing has 

 been known concerning the germ layers, &c., of these animals; and it is 

 chiefly on embryological grounds that I believe a comparison with the 

 other groups of Anthropods must be based. 



It is generally believed that the adults are in many respects degenerate 

 and adapted to a very special habitat — the abdomen has become lost, or 

 almost so, and all traces of respiratory organs are gone, the general surface 

 of the body functioning as such : also that the group is an old one, and 

 not derivable from any existing groups of Arthropods. So far we are 

 together. Without going into details, it does not seem probable that the 

 group is closely related to the Crustacea, nor very closely to the Insects. 

 Here I can only use the tout ensemble of the above facts as evidence for this 

 statement. AVe are then left to decide between an independent origin for 

 the group and an alliance with the Arachnids. If there are any special 

 reasons for an alliance with the Arachnids, I believe such facts must turn 

 the greater weight of evidence towards such a relationship. Briefly then 

 in this connection these considerations must be given: 



1. The process oi multipolar delamination to form the endoderm is, I believe, 

 common to the two groups. We have it represented in its greatest sim- 

 plicity in the majority of the Pycnogonids, while Pallene furnishes an 

 analogy to the changes which an accumulation of food yolk will cause in 

 this process, and renders a comparison with the Arachnids quite possible. 

 I will refer to Metchnikofl''s figures for Chelifer, and to Balfour's embryology 

 for the Spiders (Vol. I, page 119, Sec. Ed.). Here we read: "It apjjears 

 to me probable that at the time when the superficial layer of protoplasm is 

 segmented ofl' from the yolk below, the nuclei undergo division, and that a 

 nucleus with surrounding jjrotoplasm is left with each yolk column." Com- 

 pare Fig. 6 and 7, and see account of Pallene. 



2. The formation of an opaque area (Pallene) at the place where the 

 stomorlseal invagination appears. 



3. The early formation of meioderm at this place — the primitive cumulus 

 of Spiders. (?) 



4. The general mode of appearing of ganglia and appendages. 



5. The body cavity of the appendages and the early presence of mesoderm. 



6. The formation of endodermal pouches from the mid-gut into the append- 

 ages, these pouches containing yolk in the embryo. Compare Chelifer and 

 Spiders. 



7. The large " upper lip " of Chelifer suggests an homology with the pro- 

 boscis of pycnogonids. 



8. The first (Chelate) appendages appear at the sides of the stomodreum 

 and subsequently move forward, and are innervated from part of the supra- 

 oesophageal ganglia (brain). They will in this bear out a close comparison 

 with Chelifer (or with .\rachnids.) 



9. The lumen of the invagination of the stomodajum is triangular in 



