“TERRA NOVA ’ EXPEDITION. 
8 
still logical and simple, becomes inadequate to support the weighty conclusions that 
must be based on it. 
At this point Prof. Bouvier attributes to Prof. Carpenter and myself an opinion 
that I, at least, do not hold. He writes: “An surplus si, comme le pensent 
M. Carpenter et M. Caiman, la paire de pattes poste'rieures est une paire surajoutee 
dans les types decapodes, les orifices sexuels des Pentapycnon devraient se trouver a la 
meme place que cliez les Pycnogonum, a savoir sur les pattes de la quatrieme paire, 
alors qu’ils sont situes sur la cinquieme.” He proceeds to argue that the somite which 
has disappeared in the octopodous forms is not the fifth but the fourth, on the ground 
that the dorsal tubercle corresponding to this somite in Pentapycnon persists in 
Pycnogonum although the somite itself has disappeared. Clearly, however, this 
evidence might be read in another wav. Instead of assuming a transference of the 
dorsal tubercle from the penultimate somite of Pentapycnon to the last somite of 
■Pycnogonum, we might take the fourth pedigerous somite as homologous in the two 
genera, and assume a transference of the genital apertures from the fifth somite to 
the fourth. As a matter of fact, however, there is no evidence at all for the existence 
of individual homologies between the somites of the two genera. Bateson pointed 
out long ago the fallacy of the assumption that in variation the individuality of each 
member of a meristic series is always respected, in writing of “ an additional pair 
of legs ” I had not in mind any particular one of the five pairs. There is nothing 
to prevent us from regarding the series of somites as having been remodelled as a 
whole in passing from one genus to the other. 
In support of the contention that “ the constancy in the number of somites and 
appendages throughout the comparatively wide range of structure presented by the 
eight-legged Pycnogons strongly suggests that this is the deep-seated and, so to speak, 
‘ normal ’ plan of structure of the group ” from which the ten-legged condition is a 
secondary departure, I called attention to the parallel case of Polyartemia among the 
Branchiopod Crustacea. Polyartemia differs from the normal type of the Order 
Anostraca, to which it belongs, in having nineteen instead of eleven pre-genital trunk 
somites ; and since the number appears to be constantly ten or eleven in the other 
Orders of Branchiopoda (excluding the abbreviated Cladocera), there seems to be good 
ground for suggesting that the increased number in this case is due to secondary 
o O O O a 
specialisation. Prof. Bouvier quotes against me the authority of Dr. E. von Daday 
(1910, p. 411), who considers Polyartemia to be the most primitive of the Anostraca. 
I find nothing in Daday’s discussion of the question to lead me to change my opinion. 
He makes no mention of the position of the genital opening in comparing the 
Anostraca with the other Orders of Branchiopoda ; and his reference to the supposed 
persistence of a vestige of the mandibular palp in Polyartemia overlooks entirely the 
fact that the palp is in all cases present in the nauplius. 
It would be easy to multiply parallel instances from other groups of the animal 
kingdom, but, as Bouvier reminds us, “ il ne convient pas d’etendre a un groupe les 
