522 Reports and Proceedings — 



the posterior shield (which I demonstrated in 1866 to be the con- 

 joined thorax and abdomen) he gives the name tlwraceiron ; and to 

 the telson, or tail-spine, he applies Mr. Sj)ence Bate's name of '" pleon." 



There can be no objection to the term " cephaletron," as proposed 

 for the head in Crustacea by Prof. Owen, in contradistinction to 

 that highly specialized division of the body, the " head " in the 

 Vertebrata, but I think I have shown good grounds (in the paper 

 referred to) for assuming that the posterior shield is not merely 

 the thorax (or " thoracetron " of Owen), but the combined 

 thoracic and abdominal segments, as attested by the larval or em- 

 bryonal stages of Limulus, and by the fossil forms of the Coal- 

 measures and of the Silurian. 



I venture also to demur to Spence Bate's term " pleon " being re- 

 stricted to the tail-spine in Limulus, because it is calculated, if so 

 used, to cause considerable confusion. The term " pleon," as applied 

 to the Crustacea by its author, includes the last seven segments of the 

 body, of which the telson (if reckoned at all as being a segment) has 

 hitherto only been accounted as the ultimate joint out of the series. 



The view propounded by Prof. Owen (op. cit. p. 477) that the 

 great caudal spine in Limulus represents (either by itself, or possibly 

 with the hindmost segments of the ' thoracetron') the 'pleon' of 

 Spence Bate (or in other words, the last seven, or abdominal seg- 

 ments usually seen in other Crustacea), is based on his examination 

 of the innervation of the tail-spine. From its dissection he finds 

 that the bifid continuation of the great neural axis is divided within 

 the triangular tail-sheath (telson) into a double plexus of fine nerves 

 resembling the cauda equina of anthropotomy. In this fasciculus of 

 nerve threads the author traces nine nerve filaments, four ventral 

 and four dorsal, the ninth being the continuation of the bifid neural 

 axis. From this he concludes that the tail-spine may indicate as 

 many as four coalesced segments, which, with the three posterior 

 coalesced apodal segments of the ' thoracetron,' would account for 

 the missing abdominal series or ' pleon.' But notwithstanding my 

 profound respect and appreciation of Prof. Owen's comparative 

 anatomical studies, and his conclusions thereon, I find great difficulty 

 in adopting this view, because it appears to me that it does not accord 

 with those generally entettained regarding similar structures in other 

 orders of Crustacea, neither will it harmonize with the earliest known 

 forms of the Xiphosura, nor with the larval development of recent 

 Limulus, as made known by the researches of Packard '■ and Dohrn.^ 



Prof. Owen names the modified bifid median appendage behind 

 the mouth of Limulus the "chilaria,"^ this is, doubtless, the homologue 

 of the great metastomial plate of Fterygotus} 



the first division of the King-crab's body, and with the 'thorax' in the second 

 division." — (Owen, 021. cit. p. 463.) 



^ "The Development of Limulus polyphemus" by A. S. Packard, Jun., M.D. 

 Mem. Boston Soc. Nat. Hist., 1872, vol. xi. pp. 165-202, pi. iii.-v. 



"^ " Zur Embryologie und Morpholoo;ie des Limulus polyphemus, von Dr. Anton 

 Dohrn," Jenaische Zeitschrift, Bd. vi. Heft 4, p. 580, Taf. xiv. and xv., 1871. 



^ From x6(\apioj', a small lip (Owen, op. cit. p. 464). 



* As pointed out by me, see Fifth Keport on Fossil Crustacea, by H. Woodward, 



