344 G. H. PARKER 
Mr. P. 8. Barnhart, for much help given me while I was working 
there. 
Renilla amethystina was described originally by Verrill (64 a, 
p. 29), and the group to which it belongs was subsequently mono- 
graphed by Kolliker (72). <A detailed account of the structure 
and habits of Renilla amethystina was published by Eisen in 
1876, and the development of the closely allied species, Renilla 
reniformis, was investigated with great fullness by Wilson (’83). 
Musgrave’s experimental studies on living sea-pens appeared in 
1909. Ten years later I published a brief account of the organi- 
zation of Renilla amethystina based upon the study of the few 
specimens of this species available at La Jolla in 1916. In a 
measure this communication is a preliminary to the present in- 
vestigations, which were carried out on much more abundant 
material in 1919 and which I shall present in two papers, the first 
dealing with the circulation of water in the colony and the means 
of expansion and contraction. 
GENERAL HABITS OF RENILLA 
The majority of sea-pens are elongated colonial organisms, one 
portion of whose axis, the stalk or peduncle, anchors the colony 
in the mud or sand of the sea bottom, while the other portion, 
the rachis, is held above the sea bed and carries the polyps. 
Renilla is peculiar in that its peduncle is a relatively soft, fleshy 
structure without an axial skeleton, and its rachis is spread out 
laterally at right angles to the axis of its peduncle. When in its 
natural position the peduncle of Renilla is sunk vertically in the 
sand on the surface of which the rachis rests horizontally. The 
rachis is a kidney-shaped or heart-shaped structure whose inden- 
tation marks the region at which the peduncle is attached. The 
lower surface of the rachis, that next the sand, is devoid of polyps, 
all of which are limited to the upper surface. In consequence of 
comparison with other sea-pens, some confusion exists in the 
various accounts as to the proper designations to be used for the 
two surfaces of the rachis of Renilla. In the older terminology 
for the sea-pens, that surface which carried the zoéids was called 
ventral, the opposite one dorsal. Accordingly, in Renilla the 
