EMBRYOLOGY OF ECHINODERMS. 123 



We are indebted to Thompson for the explanation of tlie true 

 relations of the young Comatula to the present Pentacrinus and 

 the fossil Crinoids. Supposing these young to be full-grown 

 animals, he at first described them as living representatives of 

 the genus Pentacrinus ; it was only after he had watched their 

 development, and ascertained by actual observation that they 

 dropped from their stem, to lead an independent lifb as free 

 Comatulae, that he fully understood their true comiection with 

 the past history of their kind, as well as with their contempora- 

 ries. In Fig. 153, a faint star-like dot (y) may be seen attached 

 to the side of the disk by a slight line. In Fig. 154, we have 

 that minute dot as it appears under the microscope, magnified 

 many diameters ; when it is seen to be a cirrus of a Comatula, 

 with three small, Pentacrinus-like animals growing upon it, in 

 difierent stages of development. In the upper one, the branching 

 arms and the disk, with its many plates, are already formed ; 

 and though in the figure the rays are folded together, they are 

 free, and can be opened at will. In the larger of the two lower 

 buds, the plates of the disk are less perfect, and the arms are 

 straight and simple, without any ramifications, though they are 

 free and movable, whereas, in the smaller one, they are folded 

 within the closed bud. 



EMBRYOLOGY OF ECHINODERMS. 



All Radiates have a special mode of development, as distinct 

 for each class as is their adult condition, and in none are the 

 stages of growth more characteristic than in the Echinoderms. 

 In the Polyps, the division of the body into chambers, so marked 

 a feature of their ultimate structure, takes place early ; in the 

 Acalephs, the tubes which traverse the body are hollowed out of 

 its mass in the first stages of the embryonic growth, and we shall 

 see that in the Echinoderms also, the distinctive feature of their 

 structure, viz. the enclosing of the organs by separate walls, 

 early manifests itself. This peculiarity gives to the internal 



