1893.] The Organogeny of Asterina gibbosa. 433 



epithelium are derived from the coelom. The radial periheemal canals 

 and their connecting outer circular perihoemal canal are derived from 

 interradial diverticula of the coelom, one of which (PA., fig. 1) is 

 shown in the figure. The longitudinal septa in the radial canals are 

 due to the apposed walls of adjacent interradial rudiments. The 

 axial sinus (ace., fig. 1) surrounding the stone canal is the posterior 

 division of the anterior body cavity into which the pore canal opens. 

 Bury* states that in a future paper he will prove this, but he has 

 not fulfilled his promise. The stone canal is a groove in the neck of 

 communication between the anterior coelomic cavity ; it becomes con- 

 stricted off, and forms a tube opening at its distal end into the axial 

 sinus, close to the inner end of the madreporic pore canal. This 

 arrangement has been seen by Ludwig, but what he has not seen is 

 that it persists in the adult, and hence he failed to recognise the 

 rudiment of the axial sinus. 



The dorsal organ, the heart or " central-blutgeflecht," is nothing 

 more than an ingrowth of the left posterior coelom into the septum 

 separating the posterior coelomic cavities from the axial sinus (d, fig. 1). 

 It soon becomes solid. From its upper end in the adult the genital 

 rachis grows out, as Cuenotf inferred, and I have elsewhere proved.^ 



Since the genital organs are formed later, as local swellings of this 

 rachis, the ultimate origin of the sexual cells, in Asterina as in 

 Yertebrata and Annelida, is coalomic epithelium. The aboral sinus 

 surrounding the genital rachis is formed from a special diverticulum 

 of the coelom. 



As Ludwig has pointed out, the prasoral part of the larva becomes 

 converted into a special locomotor organ. It is foot-shaped, has 

 long cilia, and also functions like a tube-foot, as a temporary fixing 

 organ. Ludwig did not observe, however, that during the meta- 

 morphoses, having given up its locomotor functions, is converted into a 

 permanent fixing organ or stalk. This arrangement persists for some 

 time after the larva has acquired the adult form, for it is for a time 

 unable to use its tube feet, and when displaced from -its attachment floats 

 helplessly about. Bury has shown that the stalk of Crinoids is 

 likewise the preeoral lobe, and the free swimming larva of Antedon 

 strongly resembles that of Asterina, the main difference being that 

 the larval mouth, which soon closes in Asterina, is never formed in 

 Antedon, in which also the anterior body cavity is of less extent, 



* "Studies in the Embryology of Echinoderms," H. Bury, ' Quart. J. Mic. Sci.,' 

 1889. 



f " Contributions a I'fitude Anatomique des Asterides," L. Cuenot. ' Archives 

 de Zoologie Experimentale,' T. v. bis. 



" The Development of the Dorsal Organ, Genital Kachis, and Aboral Sinus of 

 Asterina gibbosa," E. W. MacBride, ' Zool. Anzeiger,' No. 419. 



" The Early Stages of Antedon rosacea," H. Bury, ' Phil. Trans.,' 1888, B. 



2 H 2 



