EMBRYOLOGY OF ECHINODERMS. 139 



is that of the Comatula already mentioned. Its condition in 

 the earlier stages of growth, when it is provided with a stem, at 

 once shows its relation to the old stemmed Crinoids, the earliest 

 representatives of the class of Echinoderms. 



These coincidences are still more striking among living ani- 

 mals, where they can be more readily and fully traced, and often 

 give us a key to their relative standing, which our knowledge of 

 their anatomical structure fails to furnish. This is perhaps no- 

 where more distinctly seen than in the type of Radiates, where 

 the Acalephs in their first stages of growth, that is, in their Hy- 

 droid condition, remind us of the adult forms among Polyps, 

 showing the structural rank of the Acalephs to be the highest, 

 since they pass beyond a stage which is permanent with the 

 Polyps ; while the adult forms of the Acalephs have in their turn 

 a certain resemblance to the embryonic phases of the class next 

 above them, the Echinoderms. Within the limits of the classes, 

 the same correspondence exists as between the different orders ; 

 the embryonic forms of the higher Polyps recall the adult forms 

 of the lower ones, and the same is true of the Acalephs as far 

 as these phenomena have been followed and compared among 

 them. In the class of Echinoderms the comparison has been 

 carried out to a considerable extent, their classification has 

 hitherto been based chiefly upon the ambulacra! system, so 

 characteristic of the class, but so unequally developed in the 

 different orders. This places the Holothurians, in which the 

 ambulacral system has its greatest development, at the head of 

 the class ; next to them come the Sea-urchins or Echinoids ; 

 then the Star-fishes ; then the Ophiurans and Crinoids, in which 

 the ambulacral system is reduced to a minimum. Another 

 basis for classification in this type, which gives the same re- 

 sult, is the indication of a bilateral symmetry in some of the 

 orders. In the Holothurians, for instance, there is a decided 

 tendency toward the establishment of a posterior and anterior 

 extremity, of a right and left, an upper and lower side of the 

 body. In the Sea-urchins, in many of which the mouth is out 

 of centre, placed nearer one side than the other, this tendency 

 is still apparent, while in the three lower groups, the Star-fishes, 

 Ophiurans, and Crinoids, it is almost entirely lost, in the equal 



