72 INVERTEBRATE ANIMALS. 
of the Sertularida differs from that of the Coryndda in not 
simply reaching to the bases of the polypites, but in being pro- 
longed to form a number of little cups or “hydrothecz” (fig. 
20, a, 6) within which the polypites are lodged. Each polypite 
has a hydrotheca of its own, within which it can entirely with- 
draw, and from which it can protrude its distal extremity. 
The polypites of the Sertularida have essentially the same 
structure as in the Coryuzda, and each may be compared to a 
little Hydra. Each, namely, consists of a soft contractile and 
extensile body, which is furnished at its distal extremity with a 
Vig. 20.—a Sertaluria (Diphasia) pinnata, natural size; 4 Fragment of the same 
enlarged, carrying a male capsule (0), and showing the hydrotheca (z); 4 Frag- 
ment of Campanularia neglecta (after Hincks), showing the polypites contained 
in their hydrothece (Z), and also the point at which the ccoenosare communicates 
with the stomach of the polypite (c). 
mouth and a circlet of prehensile tentacles, richly furnished with 
thread-cells. The mouth opens into a chamber which occu- 
pies the whole length of the polypite, and which is to be re- 
garded as the combined body-cavity and digestive sac. At its 
lower end this chamber opens by a constricted aperture into a 
tubular cavity, which is everywhere excavated in the substance 
of the coenosarc (fig. 20, 4). The nutrient particles obtained by 
each polypite thus serve for the support of the entire colony, 
and are distributed throughout the entire organism. The nutri- 
tive fluid prepared in the interior of each polypite gains access 
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