THE POLYZOA. 
61 
with pigment spots ; and it is now “ the dark body ” so long 
known to the student of the Polyzoa , and, as it proves, so falsely 
interpreted. In the species which I have most carefully inves- 
tigated it remains attached to the cord that connects the 
O 
polypide with the base of its cell ; it occupies indeed much the 
same position as it did when an integral part of the digestive 
system. The original tenant of the cell having run its course 
and disappeared, the bud from its own substance which it has 
left behind it enters upon a course of development resulting in 
the formation of a new polypide. It first increases in size at 
the expense of a mass of fatty globules, by which it is more or 
less surrounded ; then a small bud makes its appearance on its 
upper surface, which is gradually moulded into the rudiments 
of an alimentary canal and wreath of tentacles. Development 
proceeds step by step, until at last a fully formed zooid has 
been evolved out of the remnant of its predecessor, and the cell 
is once more in possession of a tenant. 
How often this process may be repeated we have at present 
no means of judging ; but the curious provision which I have 
just described for supplying the colony with at least a second 
generation of polypides seems to be universal amongst the 
marine Polyzoa* 
I have already referred briefly to the simple nervous system 
which has long been recognised in the individual polypides. 
Recent investigations have shown the existence of a “ colonial 
nervous system ” besides, by which the many zooids in a com- 
munity are united and brought into relation. The original 
observations of Fritz Muller have received confirmation from 
Smitt ; and I have also been able to trace the common nervous 
system in a considerable number of species. In the Polyzoa , 
such as Bowerbankia or Valkeria (fig. 1 ), in which the cells are 
set singly or in clusters on a distinct stem which is horny and 
more or less transparent, it is readily detected. This is not the 
place for details, but it may be stated generally that a nervous 
trunk pervades the stem and branches, connecting a series of 
ganglia which are situated at the joints, and communicating, 
by means of nervous filaments that sometimes originate in a 
complicated plexus, with other ganglia at the base of the indi- 
vidual cells. We have seen this structure beautifully displayed 
in a branch of Valkeria pustulosa preserved in fluid, within 
which a rich nervous plexus could be distinctly observed, giving 
origin to a multitude of most delicate threads that passed from 
it to the numerous cells composing the neighbouring cluster. 
In the calcareous species this colonial nervous system is more 
* The whole process may he regarded as a mode of gemmation. Smitt 
names the detached bud the “ groddkapsel.” 
