198 



COMPARATIVE ANATOMY. 



extends on to the arms, and gives passage to the tentacula-like feet. 

 In some the fixed condition obtains in the young only, and later 

 on in life the arm-bearing body breaks off from its stalk (Antedon, 

 Comatula). 



§ 161. 



The other series of modifications of the form of the body leads to 

 the Echinoida. The arms have altogether disappeared as indepen- 

 dent parts. In the more or less cone-shaped body of the true Sea- 

 Urchins (Desmosticha), the ambulacra extend over the greater part 

 of the surface. The ambulacral tracts form five bands, which extend 

 from the oral (Fig. 99, A o) to the opposite pole (B a) ; these are. 

 separated by as many tracts devoid of suckers (Interambulacra). 



The aboral polar area (apical 

 jj pole) is occupied by the 



grea tl y- diminishe d ant ambu- 

 lacral surface. The distribu- 

 tion of ambulacral (oral) and 

 antambulacral (aboral) sur- 

 faces of the body, which is 

 pretty equal in the Asteroi'da, 

 is thus completely altered in 

 the Sea-Urchins, for in them 

 the former is much larger 

 than the latter. If we imagine 

 a Starfish-form, in which 

 the arms have completely 

 passed into the general body 

 (cf. Fig. 96, c), then the decrease in size of the antambulacral surface, 

 and the consequent increase in that of the ambulacral, will give us 

 the Urchin-form. 



This arrangement is modified in the Petalosticha, partly by a 

 change in the relations of the mouth and anus, partly by modifica- 

 tion of the ambulacral tracts. The diminution in size of these tracts 

 is of importance. They form a five-leaved rosette on the dorsal 

 surface, and from these leaf-ends signs of a continuation of the tracts 

 as far as the mouth can be still followed out in the Olypeastridas. 



The indications of the development of the Echinoderm body from 

 a multiple of individuals are less apparent in the Holothuroi'da 

 than in the Echinoida. But the sausage-shaped body can be derived 

 from the arrangement seen in most of the regular Sea-Urchins, by 

 imagining the body to be pulled out. The oral and aboral poles in 

 each correspond, the former being distinguished by the oral, the latter 

 by the anal orifice. The antambulacral surface has altogether disap- 

 peared. In the true Holothuroi'da (Eupodia) the ambulacral and 

 interambulacra! tracts extend alternately from the mouth to the anus. 

 Some of the ambulacral tracts may however undergo an increase, 

 and the others a decrease in size, according to the varying necessities 

 of function. Thus the three ambulacral tracts on that surface which 



Fig. 99. Diagrammatic figure of a Sea-Urchin. 

 A From the oral surface. B A lateral view. 

 The ambulacra are represented by rows of 

 clots, r Radii, ir Iuterradii. o Month, a Anus. 

 The latter is surrounded by the antambulacral 

 surface. 



