MUSEUM OF COMPARATIVE ZOOLOGY. 241 
the same ganglion, and the latter shows, in the adult, no indication of 
being composed of two coalesced ganglia. So that if the accessory legs 
are not independent appendages, they must belong to the same segment 
with the palpi. According to Dohrn, however, the ganglion in question 
is represented in the larva (of Achelia) by two partially coalescent ganglia, 
and it must be regarded in the adult as representing two segments. 
In the face of these facts, it seems impossible to homologize the 
Pycnogonid appendages with those of the Arachnida unless a segment of 
the latter has been suppressed somewhere between the cheliceræ and 
ambulatory legs. The possibility of such a suppression is shown by the 
fact that in a number of Pycnogonida the process has taken place, and 
without leaving a trace in the embryological record. Thus in Pallene 
the palpi are wholly wanting, both in the adult and in the larva. 
Granting that such a suppression may have taken place, the homology 
of the Pycnogonid and Arachnid appendages is manifest. This sug- 
gestion must however be taken for what it is worth, for it is easily 
possible that the external resemblances of a Pyenogonid to an Arachnid 
are those of analogy only, and have no morphological significance. This 
is the more probable from the extreme variability of the three anterior 
pairs of appendages in position and structure.* 
One more point of interest may be noted. In Seaorhynchus the ante- 
rior pair of appendages (cheliceræ or “ antenna ”) present very decided 
sexual differences. This has not before been observed in the Pycnogo- 
nida, and furnishes another illustration of the surprising modifications 
which the anterior pairs of appendages undergo in this group. 
Following is a list of the species : — 
Pycnogonum littorale, STRÖM. 
Colossendeis angusta, SARS. 
colossea, nov. Sp. 
macerruma, nov. SP. 
Scceorhynchus armatus, nov. gen. & sp. 
* Imay take this opportunity to correct à misleading statement on page 466 of my 
** Report on the Pyenogonida of New England and adjacent Waters," in the Report of 
the United States Commissioner of Fish and Fisheries, Part VI., for 1878. The account 
there given of the innervation of the three anterior pairs of appendages, taken from Zen- 
ker’s paper, is certainly incorrect, as I have since satisfied myself by studies on the 
development of Pallene. Zenker appears to have mistaken the anterior ganglionic 
mass fora single (supra-œsophageal) ganglion, and his statements are therefore very 
misleading. There is still considerable doubt as to the exact origin of the nerves of the 
so-called antenne, but there is no doubt that the palpi and accessory legs are inner- 
vated from the first sub-cesophageal ganglion. 
VOL. VIII. — NO. 12. 16 
