510 Dr. F. A. Bather — Notes on Yunnan Cystidea. 



peristome ; the gonopore is the name frequently applied to the 

 ' fourth opening ' on the assumption that through it the gonads were 

 em'+ted, but Jaekel calls it the 'parietal pore' on the assumption 

 that it was the opening of a persistent parietal canal — the two 

 assumptions are by no means mutually exclusive. Primitively 

 these four openings lie in a straight line which probably indicates 

 the attachment of the dorsal vertical mesentery to the inner wall of 

 the theca; their order is peristome, hydropore, gonopore, periproct. 



In this series the peristome (mouth) is anterior; the ppriproct 

 (anus) is posterior. The line joining these two is the oro-anal axis. 



The theca being placed with its oral pole uppermost and with the 

 anus towards the observer, then the right and left of the theca 

 correspond with the right and left of the observer. In representations 

 of all Echinoderma, figures representing the adbral or adapical faces 

 should be placed on the paper so that the right side of the creature 

 is towards the right side of the page. Side-views of Pelmatozoa 

 should have the adoral end uppermost. 



The antero-posterior plane or anal plane passes through the oro- 

 anal axis and the vertical axis. A line drawn in this plane and 

 bisecting the vertical axis at right angles would be the antero- 

 posterior axis. This does not coincide with the oro-anal axis, and 

 the concept is rarely required. 



As regards the sagittal plane there is perhaps room for hesitation. 

 "VYhen the thecal openings all lie in the antero-posterior plane, then 

 that plane is the sagittal plane, as it would be in any symmetrical 

 animal. The plane at right angles to it and passing through the 

 vertical axis is then the transversal plane (Text-fig. 2). 



In many cystid genera asymmetry is manifested m a migration of 

 the anus and in a correlated or an independent shifting of the 

 gonopore. The oral pole being regarded as fixed, then it appears 

 that the hydropore (or madreporite) undergoes less lateral change of 

 position than the other organs. The plane tlirough the vertical 

 axis and the hydropore may therefore not coincide with the antero- 

 posterior plane; it needs a distinct name, and I have called it the 

 M plane (= madreporite plane) (Text-fig. 4). 



It seems best to limit the term 'sagittal' to its primitive 

 morphological use, available only for outwardly symmetrical or 

 almost symmetrical forms ; and for asymmetrical forms to use the 

 terms ' anal plane ' and 'M plane'. When a sagittal plane cannot be 

 fixed, then it is inaccurate to use the term ' transversal ', and another 

 term (should one be thought necessary) must be found for the plane 

 passing through the vertical axis and the peristomial extension, 

 which plane in pentamerous Pelmatozoa separates the bivium 

 (radii C D) from the trivium (radii E A B) ; it may be called the 

 peristome plane. As a rule, but not always, the plane which 

 Dr. Reed has termed ' sagittal ' corresponds roughly to this 

 peristome plane. In Pelmatozoa the peristome plane usually 

 lies approximately at right angles to the M plane, but this arrange- 

 ment is not inevitable. In the Spatangoid sea-urchins the peristome 

 plane separates the bivium (A B) from the trivium (C D E) — quite 

 a different plan; theoretically it forms an angle of 54° with the 



