2 T. H. MORGAN. 



unsolved ; the group was placed now here, now there, now 

 amongst the Crustacea and next amongst the Arachnids. Then 

 Prof. Dohrn attempted the solution from the standpoint of 

 Embryology, instituting a comparison " or even identity " be- 

 tween the Pycnogonid larva and the Nauplius, believing the 

 Pycnogonids to have diverged from the Crustacea at this point. 

 During the following t\venty : five years opinions once more vacil- 

 lated between Arachnidan and Crustacean affinities. Recently 

 Prof. Dohrn and Dr. Hoek have each independently mono- 

 graphed the group, placing the morphology of the order on a 

 very firm basis. Independently likewise they each reached the 

 conclusion that the group is to be placed neither with the Arach- 

 nids nor with the Crustacea, but that these three groups have 

 come down in parallel lines. 



The early stages of the embryology of the Sea-Spiders have 

 been practically untouched, and before any final decision as to 

 the affinities of the group is to be made, these stages in the devel- 

 opment should be known and take equal rank with Comparative 

 Anatomy in disentangling the relationships of the group. 



For many reasons the present paper attempts in no way to 

 give a complete answer from the embryological side. The very 

 great difficulties of a suitable technique had slowly to be over- 

 come, 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. 



In the summer of 1889 material for work was collected 

 at Wood's Holl, Mass. Through the courtesy of Prof. Mac- 

 Donald I was enabled to collect and study at the station of 

 the U. S. Fish Commission at this place. To Prof. MacDonald 

 I am also indebted for many other kindnesses extended during 

 my stay at Wood's Holl. Three genera of Pycnogonids, each 

 with a single species, are to be found at this place, viz. Pallene 

 empusa, Phoxichilidium maxillare, Smith (Anoplodactylus lentus, 

 Wilson), and Tanystylum orbiculare. During July, August and 

 September these are found with eggs. Pallene inhabits the 

 hydroids (Tubularia, Pennaria) on the piles of the wharves, and 

 is also common on the red sea-weeds below low-tide mark. The 

 hydroids or sea- weeds as soon as collected were brought into the 

 laboratory and worked over piece by piece. Each bunch was 



