180 ZOOLOGY 
in a thin chitinous tube, from the mouth of which the animal 
protrudes under favourable circumstances, and into which it 
withdraws in time of danger. 
The size of the colony varies a 
good deal, it may, however, 
attain a diameter of some inches, 
and may weigh a pound or more. 
The chitinous tube is a modi- 
fication of the cuticle, and is 
secreted by the epidermis of 
the body-wall. The cuticle is 
usually spoken of as the ectocyst, 
the body-wall underlying it 
being called the endocyst. The 
anterior portion of the body is 
capable of being extended beyond 
the mouth of the tube, and 
bears a _ series of tentacles 
Fic. 114.—A portion of Plumatella 
Sungosa seen in vertical section. arranged 1a: horseshoe - shaped 
ea ae lophophore. This portion can 
. Mouth of tubes. : : 
2 Cavitytoe tubes: also be retracted, and in this 
3. Statoblasts. condition the space in which 
4. Piece of wood in which the Ae 
Coleaearsieremine: the tentacles le is termed the 
tentacle-sheath. 
The body-wall, with its cuticle, is horny in Plumatella, 
gelatinous in Lophopus, and calcareous in most marine Polyzoa. 
It is often known as the zooeciwm, whilst the extrusible part of 
the organism, with the tentacular crown and the alimentary 
canal, etc., is distinguished as the polypide. 
The body-wall of Plumatella contains a layer of external 
circular, and of internal longitudinal muscles, and is lined by 
a ciliated epithelium; at its lower end it becomes free from 
the ectocyst, and by the contraction of the muscle fibres in 
this unattached region the coelomic fluid is forced forward, and 
serves to extrude the polypide. Certain muscle fibres stretch 
from the endocyst to the wall of the extrusible portion of the 
body. These have been termed the parieto-vaginal muscles, 
and serve to prevent the full extrusion of the polypide. The 
zooecia at their basal ends open into one another, and the 
