114 HERBERT E. DURHAM. 
“Kanale zum Wassergefass gehérend” by Hamann! (1, 
‘Die Echiniden,’ v. taf. xii, fig. 8, &c.) in Spatangus 
purpureus. 
Into this space or system of spaces there is free communica- 
tion, on the one hand, with the water-tube, and on the other 
with the madreporic pores; but this only occurs in certain 
forms—Spatangus (Hamann), Dorocidaris (Prouho). Hamaiun 
denies that there is any such communication in the regular 
KEchinids he investigated: this space, therefore, bears exactly 
the same position in these Echinids that the axial (perihemal) 
sinus holds in the Asterid; in fact, the one is the homologue of 
the other. The presence or absence of free communication 
with the water and madreporic tubes depends upon whether 
the embryonic developmental condition has been retained or 
lost. There is some difference in the arrangement of the axial 
sinus in the Asterid and the Echinid, for whereas in the former 
the sinus contains the dorsal organ, in the latter it is nearly 
surrounded by the tissue of that organ; that is, in the former 
the wall of the sinus has only given origin to hemal strand 
tissue along one line, whilst in the latter this tissue has been 
developed from all parts of the wall except a narrow strip 
on either side of the water-tube. If we imagined the wall 
of the axial sinus of an Asterid to contract upon the con- 
tained organ, and ultimately come in contact and fuse with 
its surface, except along the stone canal, we should obtain a 
condition closely resembling that described by Prouho in Doro- 
cidaris ; some alteration would have to be made in the structure 
of the dorsal organ at the same time, for it does not consist so 
definitely of a number of anastomosing tubular structures as it 
does in the Asterid. Furthermore, we may predict that if as 
Bury shows that in Asterids the axial sinus is derived from the 
left anterior enteroceele, careful investigation will show that the 
1 In Spatangus purpureus Hamann (loc. cit., p. 183) describes a 
space surrounding the upper end of the dorsal organ (vide also Prouho and 
Sarasin), which he considers homologous to the axial sinus of Asterids ; 
probably this has been shut off from the rest of the axial sinus by collapse 
of intercanalicular spaces during development. 
