500 j PROF. OWEN ON THE ANATOMY 
(* thoracetral") segments, in the space between the head (* cephaletron °) and pygidium 
(*pleon?), in the embryos of Sao hirsutus, Agnostus nudus, Trinucleus ornatus. These 
developmental phenomena bear a significant analogy to those observed by Newport * in 
the Julide—the successive appearance, viz., of body-segments, in the space anterior to 
the terminal, spinous, or pygidial division these also appearing at successive moults, as 
in the Trilobites. 
These, with other facts noted in the anatomical sections of tbe present paper, such as 
the fusion of the pair of cephalic ganglia, the shortness and thickness of the ‘crura’ 
connecting these with the subcesophageal mass, giving the condition of that part of the 
nervous system, as in Scorpio and Julus, as an *annular centre,” might be viewed in the 
following relation—viz., that herein Limulus manifested the more * generalized type” of 
articulate structure, in which not only Arachnidan but Myriopodal characters were asso- 
ciated with Crustaceous ones. But, in the development of Limuius, the pleon or tail- 
spine (—pygidium) was the last to appear, and, at its first budding, looked like a ninth 
segment of thoracetron. Packard, as we have shown, speaks of indications therein 
(transitory, indeed) of segmentation of the crust; and such indications I have shown to 
be more strongly and lastingly given by the nervous system. 
After formifaction and the attractive and repellant forces have produced, in the germ- 
mass, the phenomena of segmentation and vegetative repetition (as manifested in the 
similar and parallel heaps of granules, like bricks for the building), the inherited influences 
seem to overrule the polarie ones and operate in differentiating and adaptive lines, 
speedily showing the embryo-form of a Limulus; which, like that of Astacus fluviatilis, 
Palemon adspersus, Crangon maculosus, Eriphia spinifrons, Spiders, and, one may add, 
Cephalopods, goes straight to the goal of parental characters. There is no divergenee 
to a larval form enjoying for a term an active independent life. There is no metamor- 
phosis, either nauplial, zoéal, or trilobitic. 
Other representative analogies, however, can be adduced, which are plain and intelli- 
gible. Arrest the development of Limulus at the tailless stage (figs. 7, 8), and one gets 
a * Belinurus stadium.’ Stay awhile in serving the warrant, and you have the short- 
tailed palæozoie Limuloids—a * Prestwichia stadium.’ 
Segments indicated by the nerve-pairs but concealed or suppressed by the crust at the 
base of the tail-spine in Limulus, were realized in Hemiaspis limuloides (H. Wd.). The 
progress from the general to the special, from vegetative repetition to concentrative unity, 
is exemplified in the living representative of the old form discovered by Salter in the 
bed of a Silurian sea now contributing to form the county of Shropshire. The ancestral 
pleon has been almost “rubbed out" in the thousandfold generations of which the 
Salem King-crab is the heir; but the paleozoic taint sticks to the nerve-element. Or 
shall we say that Limulus, made perfect for its sphere and habits of life, must have its 
* alpen-stock " unbroken, of compact stuff without joints near the grasped end? But 
then the teleologist has to give an account of the intermediate or * evolutionary trans- 
itional' eondition of the three pleonal segments manifest outwardly, as doubtless by 
* «On the Organs of Reproduction and the Development of the Myriapoda,” Phil. Trans, vol. exxxi. 1841; and 
Owen, ‘ Lectures on Invertebrata,’ 8vo, 1855, p. 394. 
