OF THE AMERICAN KING-CRAB. : 493 
Shortly after the foregoing so-called ** Naupliusstadium," the thoracetral limbs begin 
to show; and this is termed the ** Zoéal stage." The phenomena supporting or suggest- , 
ing that phrase are, that in the Limuline larva both cephaletron and thoracetron are de- 
fined, with limbs, and that the pair of compound eyes are discernible on the former. But 
the Zoéa of the Brachyura * is framed, like the Nauplius of Fig. 11. 
the Entomostraca, for free natatory life. Its limbs are exclu- 
sively * cephaletral, and are terminally branched and ciliate. 
The thoracetral segments show no limbs; and the terminal NN j 
or *pleonal” one is bifurcate and ciliate, for assisting the VA À F 
parial limbs in swimming (cut, fig. 11). NN X^ O) | 
At the subsequent, so-called, “ Trilobite stage” (fig. 7) +, NN MAL: j 
the young Limulus has a superficial resemblance to some here CUN y 
of the Trilobites, and especially when these are at perhaps a R E 3 A 
corresponding period of development. The body of the so- >, Mg < a 
called larva e. g. of Trinucleus ornatus consists of two shield- ya A S 
like and somewhat semicircular-shaped parts joined together e AN 
by their truncate or transverse borders. The upper surface of 
the foremost, answering to the ‘ cephaletron’ of Limulus, has /) N 
also a raised median region defined from the two lateral re- 
gions—a configuration which suggested the term ‘ Trilobite.’ 
But here the resemblance ceases in the main. 
The hind division in the Trilobite is not the homologue 
of that of the larval Limulus. The *thoracetron' of Trilobites is developed, like the 
supernumerary segments added to the primary “eight” in Julus, by successive formation 
in the germinal space between the cephaletral and pleonic or pygidial divisions of the 
body. The cephaletron of the Trilobite has no articulate appendages. It is doubtful, to 
say the least, whether any were attached to the thoracetron. "What have been supposed 
to be such in that part of Asaphus platycephalusi, are not lamelliform, operculate, or 
branchiigerous at any period of the Trilobite's existence, but, if the ridged (what a car- 
penter would call * beaded’) inferior borders of the eight thoracetral segments have not 
been so misinterpreted $, are slender, filamentary, cylindrical, jointed ambulatory limbs, 
terminated by a claw ||. Under either alternative the difference is great as compared 
with the coalesced pairs of broad lamelliform articulate appendages of the thorace- 
tron of the larval as of the mature Limulus and still greater when the Trilobitic 
larva, with its pleon or pygidium for the second body-part, is compared with what 
is termed the “ Trilobitenstadium ” of the Limulus; in which stage one sees, with 
the thoracetron for the second body-segment, beneath it already developed three or 
* Anat. of Invertebrata, 1855, p. 340, figs. 138, 139. 
+ “Das Stadium welches wir jetzt betrachten wollen, können wir am Besten und Bezeichnendsten das ‘ Trilo- 
bitenstadium ” benennen.”—A. DoHzx, op. cit. p. 588. 
+ By E. Billings, Esq., F.G.S., Quarterly Journal of the Geological Society of London, vol. xxvi. pl. xxvi. fig. 1 
(May 1870) ; also by H. Woodward, Esq., F.G.S., Geological Magazine, vol. viii. (1871) pl. viii. 
$ I offer this alternative with diffidence, as I have not had the opportunity of examining the exceptional specimen. 
|| According to H. Woodward's restoration, in * Geological Magazine, July, 1871, pl. viii. fig. la. — — 
Larva or * Zoéa” of Penœus. 
