i8o 



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 

 arranged in a horseshoe -shaped 

 lophophore. This portion can 

 also be retracted, and in this 

 condition the space in which 

 the tentacles lie 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 zooecium, 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 waU 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 



Pig. 114. — A portion of Plwnatdla 

 fungosa seen in vertical section. 

 Allman. 



1. Mouth of tubes. 



2. Cavity of tubes. 



3. Statoblaats. 



4. Piece of wood in which the 



colony is growing. 



