ECHINODERMATA. 545 



Crinoidea ; and when they are present the base of the apical system is 

 said to be dicyclic, when absent monocyclic. Primary interradials occur 

 in Asteroidea, Ophiuroidea and Pelmatozoa. The oral system is completely 

 developed only in certain Pelmatozoa. In its typical form, as seen in some 

 Actinocrinidae among Palaeocrinoidea, there is an oro-central surrounded 

 by five interradial oral plates, and these in their turn by circles of oral 

 radials and interradials, the whole forming a dome, beneath which lies 

 the mouth. Apertures pass to the mouth between the orals. As a rule, 

 however, the oro-central is not formed, and the oral plates alone represent 

 the oral system. The region of the body between these two systems of 

 plates is moulded in a fashion characteristic of the various classes. In 

 Echinoidea it is spherical or compressed dorso-ventrally, and the ambu- 

 lacra and inter-ambulacra extend in parallel lines or meridians from the 

 oral to the apical area. In the Brachiate Echinoderms (Asteroidea, 

 Ophiuroidea, Crinoidea and Cystoidea] the radial meridians are prolonged 

 outwards into arms surrounding a central disc, while the interradial 

 remain more or less undeveloped. At the same time the ambulacra are 

 confined to the ventral surfaces of the disc and arms, and there is de- 

 veloped outside the apical system an area of dorsal surface known as the 

 anti-ambulacral. The skeletal plates corresponding to these different 

 regions vary much. It is of importance, however, to note whether the 

 radial water- vascular trunks are supported (i) by a system of external, 

 i. e. super-ambulacral, plates alone {Echinoidea}, or of internal, i. e. sub- 

 ambulacral, ossicles alone (Asteroidea], or of both combined (Ophiuroidea]. 

 Radial anti-ambulacral ossicles attain great importance, and form the 

 arm skeleton in Crinoidea and Cystoidea. 



The central nervous, the water-vascular and blood-vascular systems 

 form circumoral rings connected with radial prolongations. The nervous 

 elements consist of parallel fibres with interposed ganglion cells. The 

 ring and nerves lie at the base of the ectoderm cells in Asteroidea and 

 Crinoidea, but in other Echinoderms in the cutis, and then they are 

 surrounded by a cellular sheath. 'A peripheral sub-ectodermic plexus is 

 well developed in Asteroidea, and a more deeply placed plexus in Echi- 

 noidea and Plolothurioidea. The Crinoidea possess an anti-ambulacral 

 nervous system. Eyes are found in Echinoidea and Asteroidea. Auditory 

 organs in a few HolotJmrioidca. The blood-vascular system is apparently 

 mesodermic in origin. Its ring is connected, except in the Holothurioidea, 

 with radial vessels lying between the radial nerves to the outer, and the 

 water-vascular vessels to the inner side. It is also connected, except in 

 the class named, with a plexiform organ or gland the so-called heart 

 which lies in the primitive mesentery supporting the water-tube. The 

 epithelium of the plexiform vessels appears to form, at least in Echinoidea. 

 and Asteroidea, blood corpuscles, which are coloured and respiratory in 



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