866 The American Naturalist. [ October, 
refer to Bury’s discovery that the central water vascular apparatus is 
developed in three pieces, (1) the water tube, (2) an ampulla of an 
anterior enteroccele, (3) the water pore. He further promises to prove 
that the left anterior enteroceele becomes the so-called slauchformiger 
canal, here called the axial perihæmal sinus. In specimens of Cribrella 
2 mm. in diameter, I find that there is as yet but a single water pore, 
which communicates with the cavity of axial sinus; into the latter the 
free end of the water-tube opens ; thus these three spaces are in commu- 
nication with one another at a comparatively early period. Now this 
free communication may remain throughout life in many forms, as 
Cuénot proves, and as I showed in Cribrella oculata. 
Now the cavity of the axial sinus extends amongst the strands which 
form the dorsal organ; these spaces we will term intercanicular as 
distinguished from the intracanicular which are the actual cavities of 
the strands themselves, and between them there is no free communica- : 
tion, as has already been stated. 
In the dorsal organ of echinids there exist epithelium-lined cavities 
which communicate together, and with a cavity extending longitudi- 
nally along the organ; this is termed the canal aquifére annexe by 
Prouho, and the spaces Kanäle zum Wassergefiiss gehorend by Hamann 
in Spatangus purpureus. 
Into this space or system of spaces there is free communication on 
the one hand, with the water tube, and on the other with the madre- 
poric pores, but this only occurs in certain forms (Spatangus, Dorocid- 
aris). Hamann denies that there is any such communication in the 
regular echinids 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 on 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 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 hæmal strand tissue along 
our 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 contained organ, and ultimately to come in contact 
and fuse with its surface, except along the stone canal, we should 
obtain a condition closely resembling that described by Prouho im 
