MUSEUM OF COMPARATIVE ZOOLOGY. 241 



the same ganglion, and the latter sliows, 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 palin. According to Dohrn, however, the ganglion in question 

 is represented in the larva {oiAchelia) 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 chelicera3 and 

 {imbulatory 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 Pycnogonid 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 Scceorhynchus the ante- 

 rior pair of appendages (chelicera; or " antenn£c ") 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, Strom. 

 Colossendeis angitsta, Sars. 



colossea, nov. sp, 



macerrima, nov. sp. 

 SccBorhynchus armatus, nov. gen. & sp. 



• I may take this opportunity to correct a misleading statement on page 466 of my 

 " Report on the Pycnogonida 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 appeal's to have mistaken the anterior ganglionic 

 mass for a single (supra-cesophageal) 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 antennre, 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 



