The Morphology of Coeloplana. 63 
and the excretory pores, 2) the adhesive cells or colloblasts of the 
pinnate tentacles which are the unique character of the Ctenophora. 
3) the otolith and 4) the ciliated rosettes, which are identical with 
those found in the Ctenophora, even to the number of cells composing 
them. The last two characters, especially the last, must be considered 
as vestigial structures reminiscent of a previous pelagic habit. The 
otolith, the essential organ for orientation in a free swimming 
Ctenophore, we would hardly expect to find developing in a sluggish 
littoral form like Coeloplana but its presence is explicable on the 
supposition that Coeloplana is descended from a pelagic form. The 
ciliated rosettes in the pelagic Ctenophores function without doubt 
as means for hastening the absorption of digested food substances 
into the mesogloea lying between the relatively widely separated 
gastric canals. In Coeloplana with its great reduction in amount of 
parenchyma due to the flattening out of the body, and the ramifications 
of the canal system, such organs must be almost functionless. 
While the characters just mentioned have evidently been 
preserved unchanged in the adoption of a littoral habit, other 
structures have been modified in accommodation to that habit. The 
costae have disappeared. In Coeloplana the flattening of the body 
has also brought about attendant changes such as the vertical 
compression of the flaring proximal portion of the tentacle sheath 
to produce the “accessory sheath”, the much greater development of 
the muscular system, the extensive anastomosing of the gastric 
canals and the conversion of the stomodaeum into a capacious pharynx. 
A careful examination will also show how the arrangement of 
the gastric canals in Coeloplana may be merely a modification of 
the system in the Cydippida. It will be recalled that in a typical 
Cydippid the mouth opens into a long stomodaeum leading into an 
infundibulum from which branch out certain definite canals. Two 
of these, the perradial canals, lying in the tentacular plane, send 
off an interradial canal on either side and another one directed 
downwards, the stomodaeal canal, and are continued distally into 
the muscle of the tentacle as the tentacular canals. Each perradial 
canal divides in turn into two adradial canals that terminate in 
vertical expansions underlying the rows of costae. Complementary 
to the stomodaeum the infundibular canal leads upwards from the 
infundibulum to the base of the otolith, communicating with the 
exterior by the two excretory pores. 
In Coeloplana, on account of the great flattening out of the body. 
