36 



FiK. 11. 



Shortly after the foregoing so-called " Naiiplius-staclium," the thoracetral limbs begin 

 to show ; and this is termed the " Zoeal 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 Zoim of the Brachyura * is framed, like the Nauplius of 

 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 

 or ' pleonal ' one is bifurcate and ciliate, for assisting the 

 parial limbs in swimming (cut, fig. 11). 



At the subsequent, so-called, " Trilobite stage" (fig. 7) t, 

 the young Limulus has a superficial resemblance to some 

 of the Trilobites, and especially when these are at perhaps a 

 corresponding period of development. The body of the so- 

 called larva e. g. of Trinucleus ornatus (cut, fig. 12, a) consists 

 of two shield-like and somewhat semicircular-shaped parts 

 joined together by their truncate or transverse borders. The 

 upper surface of the foremost, answering to the ' cephaletron ' 

 of Limulus, has also a raised median region defined from the 

 two lateral regions — a configuration which suggested the term 

 'Trilobite. ' 



But here the resemblance ceases in the main. 



The hind division in the Trilobite (fig. 12, c) is not the homologue of that of the larval 

 Limulus. The ' thoracetron ' of TrUobites (ib. b) is developed, like the supernumerary 



Fig. 12. e 



Larva or " Zo'ea " of Pencens. 



Larvae of Tnnucleus ornatus. 



segments added to the primary ' eight ' in Julus, by successive formation in the germinal 

 space between the cephaletral (ib. a) and pleonic or pygidial (ib. c) divisions of the body. 

 The cephaletron of the TrUobite has no articulate appendages. It is doubtful, to say the 

 least, whether any were attached to the thoracetron (ib. b). "What have been supposed 

 to be such in that part of Asaphus platycephalus \, are not lamelliform, operculate, or 



• Anat. of Invertebrata, 1855, p. 340, figs. 138, 139. 



t " Bas Stadium welches wir jetzt betrachten wollen, kijimou wir am Beaten und Bezeichnendsten das ' Trilo- 

 bitenstadium ' benennen." — A. Dohen, op. cit. p. 588. 



+ By E. Billings, Esq., F.G.S., Quarterly Journal of the Geological Society of London, vol. xsvi. pi. xxvi. fig. 1 

 (May 1870) ; also by H. Woodward, Esq., F.G.S., Geological Magazine, vol. viii. (1871) pi. viii. 



