MKMOllfS (»r Till', NATIONAI- ACAIUvMV OF S( I KNCHS. 



:;ir) 



Tims l';ir our results tcml t<> provr lliiit Ilicrf :nc in tin- niihryu I lure iiairsof |iii'stoiiiiiil lnaiii 

 j;aii}lli:i: (I) llii- iiu'diaiieyi- ;;!iii^lia, (L') the lati'ial cyi' jjaii;;lia, ami (.1) the cciitnil or ifrcliral 



};aii;:lia with tlu-ir oiit};i'o\vlli, tl iiislirooiii ImmIjcs. \Vv have been altlc to dflt-ct tlicst^ in tli<- 



braiii of the adult liiiuulus, and liavt* huoii uiiahlc to liii<1 any otlin- iMijiortanl I'uridatMcntal 

 stiuctuu's in llu' lirain iiiopcr. 



Tin- .•liaii;;fs wliirh the dilJWvnt lobes of tin- brain undergo in late .inlMvunir and in early 

 lar\al life have in part been ileseiibed by Dr. I'atlen, and Inielly by Kishinoiiye, but need Inrther 

 ehnidation. The [Udbleni is a dill cult one, and we need material lor 

 it. whieh no observer has yet ajiiiarently had, viz, speeimens al'l<'r the 

 lirst molt. In the trilobile stage, a.s seen in Kishinouye's I'ig. I.">, re- 

 produeed on this page, (he medulla oreentral white tibrou.s portion (_/') 

 is ditVerentiateil I'rom the ganglionie cortex (jf c), but his figure does 

 not include the optic ganglia. 



IMs. xviii and xix lepre.sent sections of the brain of the larva. So 

 far as I have been able to make (uit from my .sections, the cerebral i> 

 ganglia grow rapidly in size, the median-eye lobes sink in below the [ 

 level of the uiii>er side, the lateral-eye ganglia ()verlying them and form- 

 ing the two lateral superior swellings of the upper surface of the brain. The median-eye lobes 

 are partly coalesced, and the nerves proceeding from them gidw together on leaving the brain, 

 only .separating at the median eyes, 'j^t thi.s jn'riod the miisliiooin ixidies have not yet been 

 develo])ed. 



XII. OU.SEKVATIONS f)N THE DEVELOPMENT OF THE ABDOMINAL APPENDAIJES, AND OF THE 

 GILL-SACS OB BRANCHI^. PLATE XX. 



The uuu.sual amount of interest attached to Limulus, and the great diversity of opinion which 

 has been, and .still is, held by zoologists in regard to its atWnities, seems mainly due to tlie fact 

 tliat this ainnial is a composite or generalized type, with most important features ]>e( iiliar to itself 

 and to its class; it also has some jioints of resemblance to the riiylloiiod Crustacea, which are 

 perhaps the most generalized and i)rimitive group of Crustacea, unless the IMiyllocarida l)e excepted. 

 Ou the other hand its embryology and .some anatomical characters ally it still more closely w ith 

 the Arachnida, so much .so that it .seems, contrary to our earlier views, not unreasonable to sup- 

 pose that the two groui)s may have had a common origin; though the Merostomata, with the 

 Trilobites, were with little doubt developed much earlier tliau the Aracbiiida, the class Podosto- 

 mata being nearer their vermian ancestors than the Arachnida. 



When we take into account the Cambrian Paradoxides and the Silurian Arethusina with their 

 very numerous (about 3(») segments, and what we know from Walcott's researches of the probable 

 nature of the legs of Calymene, Ceraurus, Acidaspis, etc., we can 

 see that the independent evoIuti<m of such Trilobites from some 

 branchiate Annelid was quite as natural and i)robable as the 

 derivation of some Protophylloi)od, or Protophyllocaridan Crus- 

 tacean fioin a branchiate Annelid worm, and that it probably 

 took place at an earlier geological epoch. 



Examining Walcott's restoration of the underside of Ctihjmvnv 

 genaria* as well as his tigures of actual sections, we notice a 

 great uniformity in all the ajjpcndiiges from the head to the end 

 of the pygidium, which is jtaralleled by that of the lind)S of Phyl 

 hipods. True, in the Trilobites there were no functional antenna- 

 and mouth-part.s, and all the lindjs, both of the head, thoracic 

 region, and r)f the jiygidium. were similar in shape, being cylindrical and .short, thus fitted for 

 walking, and bore dor.sal branchia-. 



The branchia-, says Walcott, as found in Calymene, Ceraurus, and Acidasi)is have three forms. 

 In the first form they divide a short distance from the attachment to the basal Joint of the leg and 



•Ki. XVII.— Restored section of the tliora 

 a Triloliite (Calymene). r, carapac.'; 

 cudopuilite ; «»', exoiwditc, with Ihojiilli 

 till' i'|iipo<lal or rcapiratoiy part or' tin- 

 pvnilapc.— .\n.T WaUott. 



