THE PYCNOGONIDS. 47 



In the first part of this paper I was rather inclined to believe 

 that the ventral organs had a phylogenetic significance, but still 

 I must believe their interpretation to be an open question. The 

 large size of the nuclei of these organs during their time of 

 greatest activity in forming, and the subsequent merging of these 

 nuclei (and cells) into ganglion nuclei (and cells), suggest that there 

 may be no ancestral meaning to the process, but that there may 

 be larval organs — something like neuroblasts — for the formation 

 of ganglia. And it has been suggested to me that these may fall 

 into the same category as the neuroblasts of Annelids, and may 

 present, by analogy, an intermediate step in the formation of 

 such cell-masses or teloblasts. 



Another interesting problem might be discussed if there were 

 space, viz. the appearance and subsequent loss of the third pair 

 of appendages ; and, again, the curious law discovered in the 

 sequence of the walking legs, and its relation to the number of 

 segments. When a pair appears for the first time, the number 

 of segments is six ; at the next moult the number increases to 

 eight, by the interpolation of one and the division of one into 

 two, and this method of increase is common to all of these 

 appendages. 



In thinking, too, over the remarkable series formed by the 

 different species of Sea-Spiders, where, as Dohrn has shown, 

 we can proceed from the simplest to the most complex, finding 

 amongst living forms all intermediate stages, one finds a most 

 suggestive field for speculation. Prof. E. B. Wilson has pointed 

 out to me the great perfection of this series, and I have since 

 been interested to find several other parallel cases in semi- 

 parasitic groups of animals, as, for instance, in the Copepods, 

 in the Cuninas, in the Barnacles and in the Sponges. It would 

 be interesting to discover the correlation, if any, between the 

 persistence of a perfect series of animals of a group and semi- 

 parasitism. 



[There are two other figures, XXV and XXYI, which I have 

 added to Plate VI, but these are not directly connected with the 

 second part of this paper; nevertheless, as I only obtained them 

 after the figures to the first part were printed, I have determined 

 to add them now for the sake of completeness. The first figure 

 is of the larva of Phoxichilidium at a time when it leaves the 



