INVERTEBRATE ANIMALS. 



the body. On the other hand, in the Hydrozoa, there is no 

 alimentary canal distinct from the body-cavity, there is no 

 nervous system, and the reproductive organs are in the form 

 of external processes of the body-wall. 



FIG. 77. 1. Fragment of F lustra truncata, one of the Sea-mats, natural size. 2. A single 

 polypide of Valkeria, magnified, showing the circular crown of tentacles. 3. A polypide 

 of Loplwpus crystalUnufi, & fresh-water Polyzoon, highly magnified, showing the horse- 

 shoe shaped crown of tentacles : a Tentacular crown ; & Gullet; c Stomach; S Intestine; 

 eAnus; g Gizzard; k Endocyst; I Ectocyst. 



The foregoing gives the essential structure of the polypide 

 of any Polyzoon^ but in nature this simplicity is lost. In all 

 cases in nature the primitive polypide possesses the power of 

 producing fresh zo5ids by a process of budding; and these 

 zooids remain attached to one another, so that ultimately there 

 is produced a compound growth or colony. Further, in almost 

 all the Polyzoa^ the outer layer of the polypide is more or less 

 hardened by the deposition in it of chitine or of carbonate of 

 lime. The skeletons thus formed are the parts of the colony 

 which are most familiarly known, and in the case of the com- 

 mon Sea-mats (Fig. 77, 1) they are very well known to sea- 

 side visitors, and are generally regarded as sea-weeds. Exam- 

 ined in its dead state, such a skeleton only shows a number 

 of little horny chambers or cells (Fig. 76, 1), each with a little 

 aperture. When alive, however, each of these cells was ten- 

 anted by a single zooid or polypide, capable of protruding its 



