MOLLUSCOIDA. 1876 
Examined in its dead state, such a skeleton only shows a 
number of little horny chambers or cells (fig. 95, 1), each with 
a little aperture. When alive, however, each of these cells was 
tenanted by a single zodid or polypide, 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 ” 
(fig. 95, 3), from their resemblance to the beak of a bird. The 
parts of this beak keep constantly snapping together, very much 
like the little pincer-like organs called “ pedicellarie” 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 many forms, the zodids composing the colony are 
united together by a well-developed nervous system, and are 
thus brought into organic connection with one another. 
The vast majority of the Polyzoa are fixed, and thus assume 
a very plant-like appearance. There is one fresh-water species, 
however (viz., Crvistate//a), in which the colony can creep about 
upon a flattened base very like the foot of a slug. In this 
same form, also, alone of all the Polyzoa, there is not any outer 
covering or ectocyst to the polypides. 
The Polyzoa are partly inhabitants of the sea and partly of 
fresh water, and they are thus divided into two groups which, as 
a general rule, differ from one another very much in anatomical 
structure, In most of the fresh-water Polyzoa the tentacles are 
borne upon a crescentic disc or stage (fig. 96, 3), so that the 
crown of tentacles assumes the shape of a horse-shoe. In 
almost all the marine forms, on the other hand, the tentacles 
(fig. G6, 2) are simply arranged in a circle. 
All the Polyzoa are hermaphrodite, each polypide being fur- 
nished with the reproductive organs proper to the two sexes. 
The eggs are simply liberated into the perivisceral cavity, where 
they are fertilised; but it is uncertain how the fertilised ova 
escape into the external medium. Besides true sexual repro- 
duction, and besides the power of producing colonies by con- 
tinuous budding, fresh individuals can be produced in many 
cases by a process of discontinuous gemmation. 
