258 



ma'ouT 1884. 



It niiiy, in fact, hv ilcscribiMl iis a lU'ivc-slu'iitli cxtciidinj,' prncticiiUy over 

 Ihi! wholo uiiiiiiiLl and ovcrywlicrc dircntly ciontiiiuous with tho extci-iiiil 

 epidermis, of whitjli, iiidwd, it forms tlio deupost a* -1 Hpocinlly modilicd 

 laycf. Siicli aciiiidilioii of (lie iici'voiis syHtctn tli('i'(> is i.'idepcndoiit reason 

 for rcgnrdiiif^ as a very primitive oiu' ; and 1 re^nird it as tin* type fronn 

 ■which thi! iiioie specialized nervous systems of the other lOcliinoderiiis 

 have been deiivcd. This spceiah'zation eotisist wtliiefly in w-piiration, more 

 or less ei)iiipiete, of tlic nervous system from tiie e[)i(U!rmis, in exa^'geru- 

 tion of the I'.iilial nerve bands with reduction of the intei'veniug parts of 

 tho nerve sheatii, and linally in sinking down of the radial nerve bands 

 into and through the dermis so tliiit they b(U!onie separated from tiic 

 external cplderniis by a layer of connective tissue which may, as in 

 Mchinids 'iiid some Ophiurids, be; lirndy caleilied. In Kehinids tho nerve 

 (dieath still persists as the external iiervous plexus outside the test lirst 

 described and tigured by Loven. 



I consider that in Crinoids the subepithelial bands most certainly ai'e 

 homologous with the radial or ambulaeral nerves of a stai'tish; ai.d 1 

 consider that they represent a ])art of a continuous nerve ahoath whieli 

 lias retained permanently its primitive conlinuity with tho epidermis, 

 The axial cords, some of tho branches of wliieh can bo traced into ex- 

 tremely close proximity with tho subcpitluilial bands, J regai'd as portions 

 of the antand)ulacral nerve sheath which, like the radial cords of Echini. is. 

 Ophiurids, and ILolothnrids, have lost their pi-imitivo position and shifted 

 into or through tho dermis. 



On this view tlio nervous systems of all recent grou})s of Echinoder. 

 mata can be reduce<l tf) ono plan, and furthei'nu)re, an explanation is 

 obtained of tho histological similarity or identity between the axial cords 

 and subepithelial baiuls, as well as of the very close relation, and pro- 

 bably continuity, between the two sets of stTuctnres in Antedon. 



It must be noted, however, that while this enables us to reconcile ilu' 

 Crinoid with t!ie other ]']chiiu)derm types of structure, it leaves the gap 

 between the two groups an exceedingly wide ono. Antedon, on this view, 

 is very far indeed from being a primitive Echinoderm: it is, indeed, as 

 regards its nervous system, the most highly dillerentiated of all recent 

 Echinoderms. On tho other hand, tho starfish has retained an exti'eniely 

 primitive typo of nervous system, which must probably bo regarded as 

 ancestral for all Echinoderms. 



A further point of interest concerning Antedon. that I observed durii;^ 

 my stay at iN'aples, is that not oidy may tho visceral mass be entirely 

 removed from the living animal without causing death, or indeed, any 

 apparent inconvenience, but that such sjiecimens very speedily regenei'ate 

 the wholo visceral mass. I have obtained a serii'S of specimens illustratini,' 

 the various stages of this very remarkable and extensive regeneration, but 

 Lave not yet had time to examine or describe them. 



I also devoted some time to an examination of fresh specimens ot 

 Amphioxus with tho object of ascertaining whether the spinal nerves liiive 

 single or double roots of origin. By following tho methods described by 

 Rohon, I have convinced myself of the accuracy of his description of 

 the existence of anterior spinal roots in addition to tho well-known anil 

 much more obvious posterior roots. Rohon"s attempt, however, to 

 Lomologize the anterior nerves of Amphioxus with certain of the cranial 

 nerves of the more typical vertebrates seems to me entirely devoid ot 

 justification. 



