NOTES ON L.M.B.C. ASTEROIDEA. Wat 
either side of the madreporic interradius, and this fold 
encloses a cavity known as the axial perihemal canal 
(Pl. XXXVIL, figs. 1,2and 4; Pl. XXXIX, fig. 1, aw. pe). 
Within the perihemal canal, and supported by its wall lies 
the dorsal organ, or, as I prefer to call it, the central 
plexus (cpl). 
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-hke 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 Asterias rubens, 
whose discs measure 3—4 mm. in diameter, it appears as 
a thickened band of undifferentiated cells, closely applied 
to the wall of the perihzemal 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. XXX VIII, fig. 2), and 
occupies a considerable portion of the cavity of the 
perihemal 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 
