186 INVERTEBRATE ANIMALS. 
tive organs contained within 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. 
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 zodids by a process of budding; and these 
+. 
SSS Zs) 
NAA eS 
3S 
cee 
Fig. 96.—1. Fragment of Plustra truncata, one of the Sea-mats, natural size. 2. 
A single polypide of Vadlkeria, magnified, showing the circular crown of ten- 
tacles. 3. A polypide of Lophopus crystallinus, a fresh-water Polyzoén, highly 
magnified, showing the horse-shoe-shaped crown of tentacles: a@ Tentacular 
crown; 4 Gullet ; c Stomach ; d Intestine; e Anus; g Gizzard ; & Endocyst ; 
2 Ectocyst. 
zodids remain attached to one another, so that ultimately there 
is produced a compound growth or colony. Further, in almost 
all the Polyzoa, 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. 96, 1) they are very well known 
to sea-side visitors, and are generally regarded as sea-weeds. 
