22 



ECHINODERMA GENERAL DESCRIPTION 



plane " of Cuenot. It is scarcely worth while to describe yet other 

 divergences of the sagittal plane from the M plane, such as occur 

 in Cystidea and Blastoidea. All of them are due to the imposi- 

 tion of a tertiary bilateral symmetry, obscuring the previously 

 existing secondary bilateralism that had already replaced the 

 bilateralism of the Diplewula. 



Cavities and their Contents. The cavities into which the 

 thecal cavity is divided by the ontogenetic changes above described, 

 are : (1) Gut and appendages, derived from the archenteron, with 

 mouth and anus in part produced by invagination. (2) Coelomic 

 cavities : (a) the ambulacral system, derived from the left hydro- 

 coel ; (b) the main body-cavity derived chiefly from left posterior 

 coelom, which in Pelmatozoa, Stelleroidea, and Echinoidea becomes 

 mainly adoral ; (c) the aboral body -cavity of Pelmatozoa (with 



columnal extensions), of Stelleroidea, 



,i. and of Echinoidea ; (d) the axial 



sinus of the same three classes, 

 derived from the anterior coelom, 

 running down into the stem in Pel- 

 matozoa, indirectly connected with 

 the hydrocoel through the stone 

 canal, and containing "the axial 

 organ" (p. 23); (e) a perioeso- 

 phageal sinus, sometimes subdivided, 

 is completely or incompletely sepa- 

 rated from (b), especially in Holo- 

 thurioidea, Echinoidea, and Ophi- 

 uroidea. 



The coelomic cavities are lined 

 by pavement endothelium, usually 

 ciliated, and sometimes further pro- 

 vided with special ciliated or flagel- 

 lated organs which keep the con- 

 tained fluid in motion (e.g. "urns" 

 of Synaptidae (Fig. V. 4, p. 233) ; 

 "ciliated cups" of Crinoid arms, 

 (after Geddes). i, amoeboid corpuscle especially pinnules ; free flagellate 



with granules of brown ferruginous pig- f. t.- -j \ mi. ^i j 

 ment, perhaps respiratory. 2, amoeboid Cells of Echinoidea). The fluid IS 



SS t ^&& similar to that found in the lacunar 



viren, (after Foettingert ; corpuscles of blood-vascular system " : it is SCa- 

 various shapes, with red colour supposed J . ' 



due to haemoglobin. 4 and 5 from the water, perhaps taken in through the 



madreporite, containing a variable 



amount of . albumen in solution - 



especially in the lacunar system, 

 and sometimes slightly yellowish or reddish. In it float various 

 bodies, viz. (a) amoebocytes (Fig. XIX. 1, 2, 4) capable of wandering 



Pio. XIX. 



Corpuscles of the coelomic fluid. 1 and 

 2 from the Echinoid, Echinus sphaera 



