PYCNOGONIDA. 



cicus primus); Wilson: oculiferous segment; Dohrn: das erste Rumpfsegment ; Bohm: Augenring; 

 Hoek: cephalothorax; Adlerz: cephalothorax; Hansen: f0rste Kropring; Sars: Hovedsegment (seg- 

 mentum cephalicum). 



The first segment, when viewed from above, presents a simple surface without any trace of 

 composition or articulation, and Krayer, when he nevertheless divides it into an ocular segment and 

 a first segment of thorax, has not been able to point out any trace of a cross-seam or any other arti- 

 culation, but has evidently started from the a priori reason that eyes cannot be found on a thoracic 

 part (cp. the following). If the animal, however, is seen from before, several seams or lines may some- 

 times be seen more or less distinctly, as marking the boundary of peculiar skeletal parts, originally'' 

 independent, but now united with the first segment of the thorax. Thus under the fore-edge of the 

 first segment of the trunk in Pallcnopsis plumipes the common skeletal part (metamere) of the cheli- 

 fori may be seen as a transverse band (pi. IV, fig. 3). - - To understand the first segment of the trunk, 

 it is quite necessary to follow the larval development from the embryo. It will then be seen that the 

 first and foremost chief part of the embryo is formed by the proboscis and the three pairs of embry- 

 onal limbs surrounding this latter, while the other chief part is not developed till later, the ambula- 

 tory legs and the four segments of the trunk together with the caudal segment not being partitioned 

 off at first The first chief part, most frequently with the exception of the chelifori, shrinks by and 

 by, loses its independence of the other chief part, and is, as it were, swallowed up by the foremost 

 part of this latter, the first segment of the trunk; not until this has taken place, and the embryonal 

 legs have fallen off, do the imagiual fore-limbs, palpi and ovigerous legs, spring forth on the lower 

 side of this segment, when they are developed at all. The further details of this growth will be 

 found in the following in the section treating of the larval development. 



If we suppose that the four segments with the ambulatory legs of the Pycnogonida correspond 

 with the thorax of the other Arthropoda, especially with that of the Arachnida and Insects, and the 

 first principal segment of the embryo with its three pairs of limbs likewise corresponding with the 

 head of those animals, the name of Cephalothorax (Hoek, Adlerz) for the first segment of the 

 trunk would be very good; but as I consider this comparison as wrong, or, at all events, as inde- 

 monstrable, I shall prefer another, less marked appellation, and as such I consider the one I have 

 chosen. I, for my part, think it to be most probable, or at all events possible, that the second princi- 

 pal segment of the Pycnogonida with its four pairs of ambulatory legs and the caudal segment can 

 be compared with the abdomen of the Arachnida, in which this part in its development has, or may 

 have a similar division into somites, and similar rudimentary limbs as in the Pycnogonida, cp. Locy: 

 Developm. Agelena, 1885, pi. II, fig. 9 n, and pi. Ill, fig- 13 15. The position of the genitals then 

 would also, as generally is the case, be in the abdomen, and in the processes of the abdomen, that is, 

 the ambulatory legs. On the other hand, the eyes would be placed on the fore edge of the abdomen, 

 but eyes (and peduncles in the pedunculated Crustacea) do not form a typical part of the body in 

 any animal, belonging to or constituting the head; and even if we, to avoid this difficulty, should call 

 the part of the body, in which the eyes are placed, cephalothorax, it is still in the hindmost part of 

 this segment, in the thorax, or the first somite of it that the eyes would be placed -- and farther 

 forward, to the head itself, they would never come. 



