140 BULLETIN OF THE 



careous bodies (fig. 100, op. cit. pp. 67, 68) to the " Basalplatte," of the 

 so-called " Stiihlchen," in the skin of the Holothurians, and to the "Rad- 

 chen " of the Chirodotae. He considers that the " Chirodotaradchen 

 den Basalplatten der Seesternstachel gleichzusetzen sind, gewissermas- 

 sen rudimentare Stachel darstellen, bei denen sich die ganze Ausbildung 

 auf die Entwicklung einer Basalplatte reducirt hat." 



A. Agassiz has called my attention to the resemblance of similar 

 bodies in Echinarachnius to the calcareous wheels in the Holothurians. 

 It seems probable that the stellate bodies in the young Echinarachnius 

 are the same as the '* Basalplatte " of the spine of Asterina.* It was not 

 observed that these bodies, as they first appear in Echinarachnius, bear 

 a definite relation either to the ambulacral tubes or the intermediate 

 intervals which we may suppose are the interambulacral regions. Al- 

 though a large number of plutei were examined, the number of these 

 bodies was not found to be uniform. Some plutei have five, some 

 one, others three, and many more than five, of these trifid calcareous 

 bodies. As the echinus grows older the ends of the three spurs of the 

 trifid spicule became divided or bifurcated, and even subdivided, while 

 in some cases these bodies were again subdivided. In all these cases 

 they are still enclosed in a transparent cyst or cell, similar to that 

 figured by Metschnikofi" for the " Hohlkehlen." This sheath or capsule 

 is supposed to be the outer enveloping layer, epiblast, of the spine. It 

 was of course my first impression that these rods were the beginnings 

 either of ocular or genital plates, and I turned to A. Agassiz's memoir 

 on the development of the starfish, where similar calcareous bodies are 

 found, to see if it were not possible to homologize them with the plates 

 which first appear in the Asteroidea. It was not possible to satisfac- 

 torily compare these structures, and I was then led to inquire whether 

 these structures are the beginnings of plates or of other parts of the 

 echinoid body. My observations at present have not gone far enough 

 to answer these questions satisfactorily. If these trifid bodies are the 

 beginnings of plates it cannot be stated at present whether they are 

 ocular or genital plates, and there is a doubt in my mind whether they 

 are plates of the test or spines of the same. 



PI. VII. fig. 16, is an instructive stage in the development of the sea- 

 urchin within the body of the pluteus. On the right-hand side of the 

 figure we see the ambulacral feet, am, of which there are more than 

 five, the additional having probably formed by lateral budding. On 



* Compare fig. 10, p. 69 (Ludwigop. cit.) with the trifid bodies of Echinarach- 

 nius. See also fig. 100, op. cit., a, h, c. 



