34 ECHINODERMA-GENERAL DESCRIPTION 



aboral pole in Echinoidea and the development of a special 

 terminal plate at the end of each ray in Stelleroidea afford 

 features of much diagnostic value, but of less morphological im- 

 portance. English writers have usually regarded the Asteroidea 

 and Ophiuroidea as well-defined classes. The normal forms are 

 in fact markedly separate, but the evidence of ontogeny, as well 

 as the existence of connecting links now, and the approxima- 

 tion of the two groups in Palaeozoic time, renders this view 

 difficult of acceptance, so that they are here combined in a class 

 Stelleroidea. 



Diagnosis of Echinoderma. Metazoa, coelomata, triptoblastica, 

 living in salt or brackish water, with a primitive bilateral sym- 

 metry still manifest in the right and left divisions of the anterior 

 and posterior coelom ; with a hydrocoel primitively developed from 

 each half of the anterior coelom, and connected with the exterior 

 by a water-pore ; with stereom composed of crystalline carbonate 

 of lime deposited by special amoebocytes in the meshes of a 

 mesodermal reticulum or stroma, chiefly in the integument (absent 

 only in the highly modified Pelagothuria, p. 230, and, according to 

 Koehler, in the Holothurians Stichopus pallens and S. torvus) \ with 

 gonads derived from the endothelium, apparently of the anterior 

 coelom ; total segmentation of the ovum produces a coeloblastula 

 and gastrula by invagination ; mesenchyme is formed in the seg- 

 mentation cavity by migration of cells, chiefly from the hypoblast. 



Known Echinoderma show the following features (imagined to 

 be due to an ancestral Pelmatozoic stage) : Increase in the 

 coelomic cavities of the left side and atrophy of those on the right ; 

 the dextral coil of the gut, recognisable in all classes, though 

 often greatly obscured ; an incomplete secondary bilateralism 

 about the plane including the main axis and the water-pore or its 

 successor, the madreporite, often obscured by one or other of 

 various tertiary bilateralisms ; the development of the hydrocoel 

 into a circumoral, arcuate or ring canal, che hydrocircus ; except 

 in the small (but increasing) number of known cases in which care 

 of the brood has secondarily arisen, development is through a 

 free-swimming, bilaterally symmetrical, ciliated larva, of which 

 in many cases only a portion is transformed into the adult 

 Echinoderm. 



All living, and most extinct, Echinoderms show the following 

 features (almost certainly due to an ancestral Pelmatozoic stage) : 

 An incomplete radial symmetry, of which five is usually the 

 dominant number, is superimposed on the secondary bilateralism, 

 owing to the outgrowth from the peristome of one unpaired and two 

 paired ciliated grooves ; these have a floor of nervous epithelium, 

 and are accompanied by subjacent radial canals from the hydro- 

 circus, giving off lateral podia and thus forming ambulacra, and by 



