OF THE AMERICAN KING-CRAB. 499 
that may be, or be aecepted, the pleon or telson in all Merostomes is terminally pointed 
or spinous, and would help in the movements of the animal much in the same way as 
Lloyd and Lockyer have observed it to act in Limulus. 
That this tail-spine (pleon and telson) is a serial homologue, reduced and simplified, 
of the segments, and not in the category of limbs or other mere appendages, the modifi- 
eations thereof in some of the extinct allies and predecessors of Limulus give evidence of 
weight. The argument for its appendicular grade, from time, “that it is developed sub- 
sequently to the other segments," can only apply on the assumption or supposition that 
all true segments or * somites’ of a Crustacean are simultaneously developed. The state- 
ment that the *tail-spine' is developed not only subsequently to, but ** from the dorsal 
surface” only of the body, has but the value of an unsupported assertion. If the attach- 
ment of the budding pleon of a Limulus at the stage figured by Packard (pl. v. fig. 27, 
op. cit.) were so different in its vertical relation from that of the antecedent segment as 
to support the assertion as to the limited locality of its attachment, the legitimate infer- 
ence would be that it represented a corresponding part of a body-segment. If succes- 
sively developed axial divisions of the body-segments are only to be regarded as such 
when they happen to bear true appendages, many of the segments or ‘somites’ of the 
Merostomes, besides the terminal one, must be relegated to the category of * peculiar” 
median appendages—a view which would much obscure and complicate the problem of 
determining the affinities of the Merostomata on the basis of well-founded homologies. 
Coneurring with my colleague, Mr. H. Woodward, in the views of the affinities which 
are expressed by his extended application of Dana's term Merostomata, which thus be- 
comes something more than a mere synonym of Gronovan's Xiphosura, 1 would remark, 
in reference to the relations, in time, of the latter to Pterygotus, Eurypterus, and allied 
extinet Silurian forms, that these manifest a more generalized character than do the 
Xiphosures. One cannot say that they are persistent or arrested embryonal forms or 
stages of development; for we have seen that Limulus, as soon as the germ-heaps are 
aggregated into unity or shape, assumes its concentrated character. Both families, 
together with the Trilobitide, exemplify that lower condition of the Crustacea which has 
been expressed by the term Entomostraca, in which, as Mr. Woodward has well remarked, 
the older, long and slender forms are analogous, in shape as well as in geological rela- 
tions, to the macrurous Malacostraca, and the short and broad forms to the Bra- 
chyura. lf we further indulged in suggesting that the Merostomata might be the 
ancestors of Arachnida, we might also conjecture that the Myriopods have come out of 
Trilobites; but this, at present, is not science. A superficial resemblance to the latter, 
as we have seen, is shown by the absence of the pleon in the earlier stages of the King- 
erab; but the very fact of the late appearance of this terminal division of the body, 
after all the segments, with their appendages, of the antecedent division (* thoracetron °) 
have been formed, is decisive against any real or representative resemblance of the 
embryo Limulus to the Trilobites—on the acceptance, at least, of the original and valuable 
observations by Barrande*, of the successive and later appearance of the abdominal 
+ Système Silurien du Centre de la Bohême, 4to, 1852 ; section vii. pp. 257-276, “ Métamorphoses et mode d'exist- 
ence des Trilobites.” 
9x2 
