1 10 ACriNI.K OF AKHA'I'U'OSS EX l'I.( )l,V\'ri( )\S M'MlThMncU. 



<MMt:iiii lli:tt (hey luc rciilly iihsciit. The coiilrjiclion ol tlio sprcinions 

 rciidcis il (liriiciill (o iiiMl(Ms(ini(l llicrxiicl sliiM'tmo of tlic lilainciits. 

 Tlic (>\;i AW laiiic and coiilaiii a coiisidcrahlc amount ol Ibod-yolk, 



Tlic siiiiiilicanc*' of (lie arraniicnicnl oT llic nicscMitcrics of (liis lonn 

 I liavt' clscwlicic pointed onl ('!>!). Tlic <'i,<;lit pcrCcct nu'scntoiics 

 o\id«Milly i'oirt'spond to I li(MMi;lil i'.dirnnhid nu'scntciics; tlic iin])er- 

 locl nu'scntcrios wliicli form jtaiis with adjacent ix'ili'ctoncs (1 and II), 

 arc t'vidondy tlio mesenteries wliieli eon\('rl llie oetameial into the 

 dodecameral condition with paired mesenteries. 'IMie ini])erfect ])aii's 

 VII and \'lll are se<'ondaiy mesentt'iies and arise in pairs in the two 

 iiiterniesiMiterial eliand)ers nearest tlie snUadar directives. 



If we omit pair \iii we have t li(> arrangement whicli occurs in (lonac- 

 iiiii<( (Hh)chmann and IIilj;'er, 'SS), and if tlie snitadar member of 

 l)aii' VII be omitted the condition ol)taininj4 in *SV///(>y>//()///\ ( llertwij;, 

 \S2) willappear. It seems that these two forms, toj;ether with Oractis, 

 represent three Mnks in the chain by wiiich the ancestor with twelve 

 mesenteries, all of which arose singly and bilaterally, becomes con 

 \('ilcd into the I lexact inia-, in \\hicii liie nuiscies arise in pairs and 

 radiall\. In «sV///(»/<//()///\ the oiiyinai method of formation is carried 

 o\(M' into the tbrmalion of the sini^le secondary mesentery. In Htninc- 

 I ill id the paired nH)de of formation is be^iniunj;', in Otuwtis it is 

 thoroni^ldy established, but in both these forms the bilateral mode still 

 holds, I'^inally, in IlnUampn t he mesenteries arise in i)airs and ladially. 



It will not be necessary to «^iiter into the details of this idea here, 

 since it has been treated of elsewhere in connection with some otlu'r 

 fads (MH<0. It may be well, however, to point out 1 hat there is embryo 

 loj^ical i'vidence to show that the secoiulary mesenteries of the Ilexac- 

 tinia' make theii appeaiance in the suleulo-Iatcral chambers earlier 

 than in the others, and those of the lateral chand)ers develop befori^ 

 thoseofthe snleo lateral ones (Dixon, 'S!)). a succession which exactly 

 coriesponds with the phyloyenetic development sceu iu the Protuc- 

 linia". 



Tribe HEXACTINIi^L. 



Actino/oa with six,ei.<;ht, (M' ten pairs of juM'ttH't mesenteries, whjch 

 form a piincipal cycle, and to which may be added a xaryinj;' nunjber 

 of additional cycles, perfect or imperfect, the mesenteries of which 

 «le\(>lop in |)airs and radially, aj>pearinii' almost simultaneously in all 

 the intt'rmescntcrial spaces. The h)n};itudinal nniselcs of each i)air 

 aic o!i the faces which look towaitis the intranu'senterial si)aces, exeei)t 

 in the case ol two (occasiomdly one) pairs, the directives, which are 

 at tached to the t wo (occasionally one) sii)hono}ilyphes, and whose longi- 

 tudinal muscles are on the faces which are turned towards the adjacent 

 iideiinesenterial spaces. 



The abovi' delinit ion ditfers considerably iVom that i^iven by Ileitwiii' 

 (\S2), who founded the tribe, Tlds results from the fact that I include 



