MOLLUSCOIDA. 189 



132, 2, a). These ciliated tentacles serve partly as respiratory 

 organs, and partly to set up a current of water by which floating 

 particles of food are brought to the mouth. The mouth and ten- 

 tacular crown can be partially or completely pulled into the sac by 

 means of a muscle which is fixed to the gullet (2, g). The mouth 

 leads into a gullet, and that again into a stomach, sometimes with a 

 muscular gizzard between. From the stomach proceeds an intestine 

 of variable length, which terminates by a distinct anus at the upper 

 part of the sac (2, c). On one side of the gullet, between it and the 

 anus, is placed a single nerve-ganglion (d). Distinct reproductive 

 organs (//) are also present, and the whole cavity of the sac is filled 

 with fluid. From the above description it will be evident that the 

 typical polypide of a Polyzoiin dilTers from the polypite of a Hydro- 

 zo'mi in having a distinct alimentary canal suspended freely in a 

 body-cavity, in having both a mouth and vent, in having a distinct 

 nervous system, and in having the reproductive organs contained in 

 the body. 



The foregoing gives the essential structure of the polypide of any 

 Polyzoon, but in nature tliis simplicity is lost. In all cases in nature 

 the primitive polypide possesses the power of producing fresh zooida 

 by a process of budding ; and these zotiids remain attached to one 

 another, so that ultimately there is produced a compound growth 

 or colony. Further, in almost all the Poh/zoa, the ectocyst or 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 common Sea-mats (fig. 131), they are very well known to 

 seaside visitors, and are generally regaided as sea-weeds. Examined 

 in its dead state, such a skeleton only shows a number of little 

 horny chambers or cells (fig. 132, 1), each with a little aperture. 

 "When alive, however, each of these cells was tenanted by a single 

 zooid or polyjnde, capable of protruding its ciliated head from the 

 aperture, and of again retiring within it if alarmed. The skeleton 

 is, in some cases, furnished with curious organs, which are known 

 as "bird's-head processes" {avicularia, fig. 132, 3), from their re- 

 semblance to the beak of a bird. The parts of this beak keep 

 constantly snapping together, very much like the little pincer-like 

 organs called " pedicellarite " in the Sea-urchins and Star-fishes ; but 

 it is difficult to see what service they perform. They continue their 

 movements long after the death of the polypides, and this appears, 

 in some cases, at any rate, to be due to a peculiar system of nerves 

 known as the " colonial " nervous system. In addition, namely, to 

 the single ganglion with which each polypide is furnished, it has 

 been shown that, in some forms, the zooids composing the colony are 



