THE DEVELOPMENT OF THE CRINOIDEA. 503 



tissue as occupies the cavity of the central tubercle, and is 

 continued throughout the ossicula of the calyx and arms, is 

 the proper central organ of the nervous system; founding 

 this opinion partly upon the fact that, when this mass is irri- 

 tated in a living Antedon, a sudden contraction of all the 

 muscles of the arms takes place, and partly upon the distri- 

 bution of the ultimate ramiHcations of the axial tissue in the 

 arms. Greef, on the contrary,^ affirms that all these tracts can 

 be injected, and retains the name of " heart " for the cavity 

 of the centro-dorsal tubercle. 



The perisoma of the oral surface of Comatula exhibits a 

 great number of minute circular pores, with thickened cellu- 

 lar margins. Greef has discovered that these are the external 

 apertures of canals, with ciliated walls, which open into the 

 body-cavity, and readily allow fluids to pass into, or out of, 

 that cavitv. 



Each mature ovary of Ajitedon has a distinct aperture, 

 through which the ova are discharged, and to which they ad- 

 here for some days like bunches of grapes. The testis devel- 

 ops no special aperture, but the spermatozoa appear to be 

 discharged by dehiscence of the integument. 



Since the discovery by Vaughan Thompson that Comatida 

 passes through a Pentacrinoid larval condition, the develop- 

 ment of the free Crinoids has been the subject of various in- 

 vestigations,'* and the following results may be regarded as 

 established : 



Complete ye'k-division takes place. The morula acquires 

 an oval form, and develops four hoop-like bands of cilia, 

 with a tuft of cilia at the hinder end. Between the third and 

 fourth bands of cilia, counting from the anterior end of the 

 Echinopasdium, the blastoderm becomes invaginated, and 

 gives rise to an archenteron. In the interspace between this 

 blind sac, the wall of which is the hypoblast, and the epiblast, 

 constituted by the rest of the blastoderm, a mesoblast com- 

 posed of reticulated cells makes its appearance. The blasto- 

 pore closes, while the archenteron detaches itself from its 

 attachment to the posterior ventral face of the larva, and be- 

 comes connected with an oesophageal involution formed at its 

 anterior end. Tlie archenteron next throws out three diver- 

 ticula, of which two are lateral and one is ventral. The lat- 



1 " Ueber das Herz der Crinoideen" (" Marburg Satzunersbericbte," 18T6\ 



2 5^^ Wyx-ille-Thompson ("Phil. Trans.," 1865^, Metschnikotf ("Bulletin 

 de I'Acad. Imp. des Sciences de St.-Petersbourg," 1871), and especially GOtte 

 (" Archivfar Mikroskopische Anatomie," 1876j, 



