SUB-BRANCH I 



I'KI.MATOZOA CRINOIDEA 



123 



Apart from this constitution.-!! din"crence. thr Kdiin- u dist inguishcd 



from tin- Coelenterates by the presence of a true- digestive canal, a vascular 

 VStem/and water \aseular apparatus : by a more perfectly <! vclojic. I nervous 

 system ; and liy an exclusively sexual mode of reproduction. 



The most striking ditl'erencc consists clearly in the nature and composition 



of thr \oskeletoll ( Kig s - l' 1 7. I 1 1 S ). All of the pi plM, .-| ii 1 1'-, a II' 1 



other indurations an- >ecivteil in the integument (dermal connective tissue), 

 and are all composed of microscopically small calcareous liodies embedded in 

 organic matter. As these liodies are united to form perforated hori/ontal 

 laminae, which are connected with one another by \ertieal pillars, the result is 

 a minutelv reticulated structure, highly characteristic of all Echinoderms. 

 During foili>ation the interstices are commonly infiltrated with lim- 

 Inmate, so that the whole structure is transformed into calcite, exhibiting un- 

 mistakable rhombohedral cleavage. Kach plate, joint, and spine of a sea-urchin, 

 Martish, or crinoid lieha\es mineralogically and optically like a single calcite 

 crystal. >. 



All the Kchinoderms are marine. In the classification proposecMvf Haeckel 

 thcv are divided into three primary groups comprising the following classes : 



Sub-branch A. 



/"Class 1. Crinoidea. 

 Pelmatozoa. - 2. 



Sab-branch B. Asterozoa. 



Suh-hiuncli ('. Echinozoa. 



I 3. Blastoideu. 



( Class 1. Opkiuroidea 

 \ 2. Asteroidea. 



(Sea-lilies.) 

 (( 'ystideans.) 

 (Blaatoids.) 



(Brittle stars.) 

 (Star-fishes.) 



TClass 1. Echinoiden. (Sea-urchins.) 

 \ 2. Holotliin'i'il,l>,i. (Sea-cucumbers.) 



SUB-BRANCH A. Pelmatozoa. Leuckurt. 



The a are Echinoderms which, during the whole or at least the" 



earlv poll ion of their existence, are fixed liy a Jointed, flexible stalk, ol' ale 

 attaehed ly the dorsal or aloral surface of the body. The principal viscera are 

 enclosed in a lnirsifonn, cup-shaped, or spheri-al test ('"///.'), which is composed 

 of a M-stem of calcareous plates ; and on the upper surface of which are situated 

 lioth the mouth and anus, as well as the ainlmlacral or food grooves conducting 

 to the mouth. Asa rule, jointed flexible arms spring from the distal ends of 

 the ambulacral grooves around the margin of the calyx; sometimes, however, 



arms are wanting, the amluilacral areas licing extended down the >ide> of the 

 cal\-\, and Keset on l>oth sides with ji-nnules (Blastoidea). The inferior (dorsal, 

 al>oral) portion of the calyx is composed of a single or douMe series of lasal 

 plates, \\hieh either rest directly upon tin- stalk, or are grouped aliout a rentro- 

 dorsal plate. 



The /'ilni'ifm.Hi are diviiled into three classes: Crinoideii, C>/.-f"i<f<", and 

 Blasl>'i>l'-ii. nf thc.M-, only the Crinoids are 1'epresented by a few existing 

 genera ; the others are wholly extinct, and are confined, moreover, to the 

 hilaeo/.oic rocks. Although tlie Cystideans possess a less highly specialised 

 organisation than the others, and probably represent the common ancestral 



