326 ARTHUR WILLEY. 
tentacles or pinne, arranged somewhat irregularly, but in a 
single series. Like the tentacles of Coloplana, described by 
Kowalevsky, they are strictly comparable with the tentacles of 
a Cydippid. Both the tentacles and their pinne are quite solid, 
being completely filled up with a muscular core. Within the 
body each tentacle is enclosed within a hollow sheath which 
opens to the exterior at the end of a small papilla at the 
base of the marginal bay. When retracted, therefore, the 
tentacles form a median axial skeletal support for the body, 
interrupted in the middle region of the body by the aboral 
sense-organ. 
The aboral surface of the body may be at once called the 
dorsal surface, and the oral the ventral surface. 
The possession of a relatively wide Planarian-hke skirt not 
only permits Ctenoplana to crawl about on firm surfaces, but 
enables it also to attach itself, in a highly characteristic 
manner, to the surface-film of water by its ventral surface. 
In this position it greatly resembles Planarians, which are also 
fond of assuming the same position. When lying thus attached 
to the surface of the water the round central oral opening can 
be seen. The mouth can be protruded so as to form a slight 
cone. 
When swimming, Ctenoplana brings the two halves of the 
skirt together so as to form a bell-shaped, or better, a Pilidium- 
shaped structure which progresses very rapidly by means of 
the ctenophoral ‘plates. In swimming, the aboral pole is 
directed forwards as it is in the Ctenophora. 
The ctenophoral apparatus consists of eight small oval 
plates, placed four on each side of the tentacle axis.‘ Across 
each plate run six or seven shallow grooves, from which the 
long cilia arise (figs. 1 and 5). The cilia of each groove appear 
in section to be united usually for some distance from their 
base, and then to separate out into the individual cilia (fig. 5). 
The ctenophoral plates alternate with the lobes of the central 
gastric system (figs. 1—3). I only had a fleeting view of the 
1 The line joining the bases of the tentacles may be called the “ tentacle 
axis.” 
