20i 



COMPAKATIVE ANATOMY. 



which form the boundaries of the body, are attached. In the young 

 stages of the Comatulaa, a simple knob-like piece (centro-dorsal) 

 unites the skeleton of the stalk with the body. Radial jointed 

 pieces, which are continued into the joints of the arms, are attached 

 to the central piece. The ambulacral groove extends along the 

 dichotomous branches of the arms (Pentacrinus), as well as along 

 the lateral appendages (pinnulae of Comatula) which are set alter- 

 nately on either side of the arms. The groove becomes united with 

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

 the cup-shaped body as far as the mouth. Deposits of calcareous 

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

 which remains soft and covers over the skeleton. 



§ 



166. 



The differences in the dermal skeleton of the Echinoida, and the 

 consequent changes in the form of their body, as compared with the 

 Asteroida, are chiefly due to the calcification of the oral (ventral) 

 perisome, that is of the portion which covers the ambulacral groove 

 and the soft parts which lie in it, and which is permanently soft 

 in the Asteroida. In the place of the articulated joints, there are 

 plates which are calcified externally, and are connected with the 

 body in various ways. 



In the Desmosticha the portion which corresponds to the dorsal, 



or aboral pole of the Starfish is a 

 small surface, marked off by small 

 loosely-articulated calcareous plates, 

 placed excentrically to the anus (Fig. 

 103, x). This surface, which occupies 

 the centre of the so-called apical pole 

 of the Sea-Urchin, is surrounded by 

 larger calcareous plates, which carry 

 the orifices of the genital organs, the 

 genital plates (</) ; one of these is the 

 madreporic plate (m). Five pieces 

 (intergenital plates, ig) are attached 

 to, and partly intercalated between 

 these, and from the former five rows 

 of paired plates extend to the oral 

 surface. These are traversed by fine 

 pores, by means of which the suckers 

 communicate with the interior. These 

 are the ambulacral plates (a), which 

 make up the ambulacral areaa. The 

 ambulacral rows of the calcified peri- 

 some of the Sea-Urchins are homo- 

 logous with the permanently soft 

 perisome of the Starfishes, which covers over the ambulacral groove 

 on the ventral surface of the arms. The rows of non-perforated 



Fig. 



Ech 



103. Apical pole of an 

 inns. a Ambulacral avea\ 

 * Interambnlacral arese. g Genital 

 plates, ig Intergenital plates, to A 

 genital plate with the characters 

 of a madreporic plate. x Anns 

 placed in the apical area surrounded 

 by the genital plates. The knobs 

 of the plates are figured in only one 

 ambulacral and one interambulacral 

 area ; in the former the pores also, 

 which are omitted from the other 

 four, are indicated. 



