PYCNOGONIDA. 



males -- tons les trois paires), les segments thoraciques se developpent snccessivemeiit an bord poste- 

 rietir du corps, et aussitot nn nouveau segment forme, nne paire de pattes se montre egalement 

 comme excroissances laterales de ce segment. Quand qnatre paires de pattes se sont ainsi developpees 

 anx quatre segments thoraciques (notons que 1'ordre de developpement des pattes correspond tout a 

 fait a lenr rang dans le corps de 1'animal adnlte), 1'excroissance terminale se change en tin abdomen 

 pins on moins rudimentaire. 



Morgan has given a little series of essays on the Pycnogonida, of which especially the last 

 one may be mentioned, entitled : A Contribution to the Embryonalogy and Phylogeny of the Pycnogonids 

 (1891). In this essay Morgan gives the development of Phoxichilidium maxillare |= femoratum 

 Rathke?], of Pallene empnsa, and of Tanystyhim orbicular e. He has more than his two above men- 

 tioned predecessors paid attention to the first development of the embryo, an examination that Kroyer 

 did not enter upon at all, and gives furthermore a very handsome series of the developmental stages 

 of the larva, especially the larva of Tanystylum; on the other hand I do not think his represen- 

 tation of the larval development of Pallene empusa to be correct. In his introduction I.e. p. 2 Morgan 

 says: For many reasons the present paper attempts in no way to give a complete answer from the 

 embryonal side. The very great difficulties of a suitable technique had slowly to be overcome, and 

 the time at command prevented a detailed description of the different organs arising from the germ- 

 layers, so that much remains that might be done, but nevertheless his essay is a very important 

 advance in our knowledge of the development of these animals, as also his representation of the 

 structure and development of the eye in the Pycnogonida is rather exhaustive. 



Passing now to my own description of the larval development I have to begin with the usual 

 complaint of not having had fresh material at my disposal; but on the other hand the Ingolf-Expe- 

 dition has brought home so rich a material well preserved in spirit that I suppose I shall be able to 

 give a more detailed and continuous description of the different stages of development in the larva. 

 I have been able to follow the development for a shorter or longer way in a considerable number of 

 Pycnogonida, belonging to the different families and genera as Nymphon grossipcs, Sluitcri, clcgans, 

 longitarse, robustum, spinosum, macronyx\ Paranymphon spinosum 1 ); Zetes (Eurycyde) hispidus; Pseudo- 

 pallene circularis and spinipcs; Pallene hastata and brcvirostris; Phoxichilidium femoratum; Pycnogonum 

 littorale, altogether 7 genera with 15 species. The species, the development of which I have most 

 complete, are Nymphon grossipes, N. robustum and Pseudopallene circularis, of which three species the 

 first and the last are those that have been particularly examined by Kroyer; but besides corrobo- 

 rating most of his statements and drawings (I have partly examined his original pieces) I have also 

 been able to increase and partly to correct some of them, which corrections especially apply to Pseu- 

 dopallene circularis. 



The segmentation, yolk-division, of the Pycnogonid ovum is complete, some- 

 times equal, sometimes unequal. 



For the correctness of this thesis I must refer to Morgan, Contrib. Embryol., 1891, and I 

 have nothing to add. It is, I think , to be supposed , as Morgan does 1. c. p. 23 , that the difference 

 between equal and unequal segmentation, which latter is also continued in the difference between 



') In pi. II, fig. 22 24 are wrongly called spinipes in stead of spinosum. 



