NOTES ON L.M.B.C. ASTEBOIDEA. 143 



either side of the madreporic interradius, and this fold 

 encloses a cavity known as the axial perihsemal canal 

 (PL XXXVII, figs. 1, 2 and 4 ; PL XXXIX, fig. 1, ax. pc). 

 Within the perihaemal canal, and supported by its wall lies 

 the dorsal organ, or, as I prefer to call it, the central 

 plexus (cjjl). 



Discovered long ago by Spix, the central plexus and its 

 homologue in other groups of the Echinodermata has been 

 repeatedly described, and its function speculated upon. 

 Tiedemann, with a nearer approach to what I believe to 

 be the truth than some of his successors, regarded it as a 

 heart. Greeff described it as a gill-like organ, Hoffmann, 

 a little latter propounding a view which still has its 

 adherents, viz., that it is a glandular body. Later still 

 Teuscher, followed by Ludwig, revived Tiedemann' s view 

 that it is a heart, but in his later papers Ludwig discards 

 the term " heart " substituting for it the more appropriate 

 term " central plexus." The organ presents a very similar 

 appearance in the four genera of Asteroidea in which I 

 have examined it. In young specimens of Aster ias ruhens, 

 whose discs measure 3 — 4 mm. in diameter, it appears as 

 a thickened band of undifferentiated cells, closely applied 

 to the wall of the perihsemal canal (PL XXXVII, fig. 1, 

 cpl). In very slightly larger specimens, however, I find 

 it to have assumed the adult condition. At its aboral end 

 it is more or less lobulated (PL XXXVIII, fig. 2), and 

 occupies a considerable portion of the cavity of the 

 perihaemal canal. Gradually tapering towards its oral end, 

 it becomes continuous with an oblique perforated septum 

 about which I shall have more to say shortly. Examined 

 by means of thin sections the organ is seen to consist of 

 anastomosing tubular strands, the walls of which appear 

 in transverse sections as an exceeedingly thin membrane 

 (PL XXXVII, fig. 2; see also the figures illustrating 



