98 



INVE]{TEI!I!ATE ANIMALS. 



sides of the body. In :-M Ecliinodeims thei'e is a water-vascular 

 sj'stem of tubes, which i.s tenued the "aiubulacral system," which 

 generall)' communicates with the exterior, and which in most cases 

 is used in locomotion. An alinientary canal is always present, and is 

 always comjjletely shut oil' from the ^liueral cavity of the body. There 

 are always distinct org.ins of rcjiroihiction, which are almost always 

 jilaced in different individualw, .so that the sexes are distinct. The ner- 

 vous system is in the form of a ling surrounding the gullet and send- 

 ing branches in a radiating manner to different parts of the body. 



One of the most characteristic j)oints about the Echinoderms is 

 their mode of development. In most cases, namely, the embryo is 

 a little ov:d body covered all over with cilia ; but in the course of 

 growth the ciiia become restricted to transverse bands or to definite 

 outgi-owths of the body. The enibiyo is now often somewhat coni- 

 ad or pyramidal in shajje, with long prr>jecting processes, and may 

 be sup|iorted by a provisional inteiiial .skeleton (tig. 60). An ali- 



Fis- fin.— Lan-i of /■-V/in,//,.. (.\rtfr ,T. Miillrr.) .\, A, Front ann.'i wiili tliriv iiitenial 

 skeleton; F, F, Aioi.^ ..f the liioiilli - in-o.rss ; H. roslorioi- sicl.vaviiis : a JlolitJi ; 

 r.' OCsoiili.aKiis : '' Stninach ; h' IiiirsI iiir ; ./ Ciliati'.i bands ; // Cili.atert Ll>aulct,s ; 

 i; Disc of tlie future Krhinu.^. 



ment.iry canal with a distinct iiioiUh and anus is present, and the 

 eriil)r>T) is thus divided into t\\*o bil;iter;i]ly synmletric^al halves, 

 l^liis singular en]b]\o was (^)iigiiialiy desi.-rilied, in the case of the 

 Si'/i-urchin.s, ,-is an inili'])cndi'nt ;inini.-d, uiidi'i' the name of " I'/ntcys." 

 Tho spo.-i.'il jioculi.-iiitv of tlie finilicr develojinient of the embryo is 

 thol the :wlult iiiiirii;d is ultiin,'it"ly di'Veloprd out of a mass of active, 

 g)'o\\in;_: |o-ot<j|jl,'ism wliicli apjicars on one side of th'j stomach of 



