204 



COMPAEATIVE ANATO]MY. 



whicli form tlie boundaries of tlie body, are attacbed. lu tbe young 

 stages of tbe ComatulEe, a simple knob-like piece (centro-dorsal) 

 unites tbe skeleton of tbe stalk vntb tbe body. Kadial jointed 

 pieces, wbicb are continued into tbe joints of tbe arms, are attacbed 

 to tbe central piece. Tbe ambiilacral groove extends along tbe 

 dicbotomous brandies of tbe arms (Pentacrinus), as well as along 

 tbe lateral appendages (pinnulte of Comatula) wbicb are set alter- 

 nately on eitber side of tbe arms. Tbe groove becomes united witb 

 tbe groove of tbe next arm, and passes along tbe ventral surface of 

 tbe cup-sbaped body as far as tbe moutb. Deposits of calcareous 

 plates are embedded in all parts of tbe portion of tbe integument 

 wbicb remains soft and covers over tbe skeleton. 



§ 166. 



Tbe differences in tbe dermal skeleton of tbe Ecbinoida, and tbe 

 consequent cbanges in tbe form of tbeir body, as compared witb the 

 Asteroida, are cbiefly due to tbe calcification of tbe oral (ventral) 

 perisome, tbat is of tbe portion wbicb covers tbe ambulacra! groove 

 and tbe soft parts wbicb lie in it, and wbicb is permanently soft 

 in tbe Asteroida. In tbe place of tbe articulated joints, tbere are 

 plates wbicb are calcified externally, and are connected witb tbe 

 body in various ways. 



In tbe Desmosticba tbe portion wbicb corresponds to tbe dorsal, 



or aboral pole of tbe Starfisb is a 

 small surface, marked off by small 

 loosely-articulated calcareous plates, 

 placed exceutrically to tbe anus (Fig. 

 103, cv). Tbis surface, wbicb occujDies 

 tbe centre of tbe so-called apical pole 

 of tbe Sea-Urcbin, is surrounded by 

 larger calcareous plates, wbicb carry 

 tbe orifices of tbe genital organs, tbe 

 genital plates (g) ; one of tbese is tbe 

 madreporic plate (m). Five pieces 

 (intergenital plates, /;/) are attacbed 

 to, and partly intercalated between 

 tbese, and from tbe former five roAVs 

 of 23aired plates extend to tbe oral 

 surface. Tbese are traversed by fine 

 pores, by means of wbicb tbe suckers 

 communicate witb tbe interior. Tbese 

 are tbe ambulacral plates (a), wbicb 

 make up tbe ambulacral are^. Tbe 

 ambulacral rows of tbe calcified peri- 

 some of tbe Sea-Urcbins are bomo- 

 logous witb tbe permanently soft 

 perisome of tbe Starfisbes, wbicb covers over tbe ambulacral groove 

 on tbe ventral surface of tbe arms. Tbe rows of non-perforated 



Fig. 103. Apical pole of an 

 Echinus. a Ambulacral arese. 

 i Interambulacral arcEe. g Genital 

 plates, ij Intergenital plates. mA 

 genital plate -n-ith. the characters 

 of a madreporic plate. .r Anus 

 placed in the apical area surrounded 

 Ijy the genital plates. The knobs 

 of the plates are figui-ed in only one 

 ambulacral and one interambulacral 

 area ; in the former the pores also, 

 which are omitted from the other 

 four, are indicated. 



