HOMOLOGIES AND AFFINITIES OF ECHINI. 757 



seems, therefore, hardly justifiable, in comparing Holothurians and Spatan- 

 goids, to suppose the latter turned o60 J round a vertical axis before we can 

 compare them homologically ; the difference in the constitution of the 

 actinal surface depends entirely upon the position of the actinostome, which 

 may be more or less anterior in Spatangina, or more or less posterior in 

 Clypeastroids, or central in the Desmosticha, while it is marginal, or rather 

 terminal, in Holothurians. In this connection we should remember that in 

 the young of many Holothurians the actinal surface is formed, as in Desmos- 

 ticha, of an equal portion of all the ambulacra, since their young creep upon 

 their actinal gill tentacles, up to the time the ambulacral tentacles proper 

 are developed. 



The ambulacral system, the genital organs, the jaws, as well as the plates 

 of the test, are arranged radially round a vertical axis ; but even in these 

 systems the plan of radiation is somewhat modified by a tendency to a 

 bilateral symmetry in some of the groups. The alimentary canal has not a 

 radial arrangement in any of the Echinodenns. The position of the anal 

 system in the Clypeastroids and in the Petalosticha gives us two poles of an 

 axis, which has been taken as the anterior axis in Spatangoids and in Clype- 

 astroids, and on both sides of which there is a marked tendency to a bilat- 

 eral arrangement in the ambulacral system. This axis becomes a vertical 

 axis in the Desmosticha. 



The atrophy of the genital organs in Petalosticha, by leaving either four, 

 three, or even only two genital glands, tends to produce a bilateral arrange- 

 ment in the genital organs, just as the specialization of the odd anterior 

 ambulacral brought out more strikingly the bilateral tendencies of the 

 petaloid ambulacra in the Petalosticha. But in spite of this tendency to a 

 bilateral arrangement in the ambulacral and genital systems of the higher 

 Echini, we cannot fail to be struck with the fact that the ambulacral system 

 which the Echinodenns have in common with the Acalephs and Polyps is 

 arranged according to the plan of radiation fully as clearly as in the other 

 great classes of this branch of the animal kingdom. It cannot be denied 

 that there are many excellent reasons for a closer association of the 

 Acalephs and Polyps than either of them and the Echinodenns, yet until 

 we can show that the radial ambulacral system of Echinodenns is not clearly 

 homologous to the radial ambulacral system of the Acalephs and Polyps, 

 we must retain these three classes in the same great branch of the animal 

 kingdom. 



