ON FRESHWATER POLYZOA. 307 
‘ ANATOMY. 
Definition of terms.—The old notion, which, by mistaking the zoological 
rank of the Polyzva, erroneously referred them to the class of the Polypes, 
caused the same terms to be applied to them which were also used to desig- 
nate the various parts of the true polypes. The-recognition, however, of a 
type of structure in the Polyzoa totally distinct from that of the Polypes 
proper, necessitates a change in the terminology employed in their description. 
On these grounds I have ventured to substitute some new terms for those 
previously used; while our increased knowledge of polyzoal structure neces- 
sitates the use of certain additional terms of which we have no representatives 
in the descriptive terminology of previous authors. For the term Polype, 
therefore, originally applied not only to the Anthozoal radiata, to which its 
use ought to be confined, but also to the retractile portion of the Polyzoa, I 
have substituted in the following Report that of Polypide*. ‘To the common 
dermal system of a colony, which, as well as the solid basis of the true Polypes, 
was formerly known under the names of Polypary and Polypidome, I have 
applied the term Caenecium+. The ccencecium is composed of two perfectly 
distinet tunics ; to the external I have given the name of Eetocyst{, and to the 
‘internal that of Hndocyst§. The sort of dise or stage which surrounds the 
mouth and bears the tentacula, I have called Lophophore||. The Perigastric 4] 
space is the space included between the walls of the endocyst and the ali- 
mentary canal. 
The terms now enumerated are such as I believe the nature of the subject 
strictly requires. I am fully aware that the changing of an established ter- 
minology is highly objectionable where it can possibly be avoided, but in the 
present case, where the very same terms are in two different classes of animals 
applied to organs in no respect homologous, the purposes of a rigidly scien- 
tific description can, I believe, only be served by some such change as that 
which I have here ventured to introduce. 
I. Organs for the Preservation of the Individual. 
A. Dermal system.—The Polyzoa are all compound animals, and by the 
expression, Dermal system, I intend to be understood, the Canecium or 
common connecting medium of the colony. It is formed of a number of 
little chambers or cells organically united, in each of which is contained a 
polypide, and consists of two portions, which must be carefully distinguished, 
an internal tunic, soft, transparent and contractile (the endocyst), and an 
external investment (the ectocyst), which varies greatly in texture and form 
in the different genera. The endocyst lines the interior of the cells, and 
when it arrives at their orifice would protrude beyond the ectocyst, were it 
not that it here becomes invaginated or inverted into itself, and then termi- 
nates by being attached round the base of the tentacular crown; during the 
exsertion of the polypide it undergoes eversion, which, however, in all the 
freshwater species is but partial ; a portion of the endocyst, as we shall after- 
wards more particularly see, remaining in a permanently inverted condition, 
in this respect differing remarkably from the marine species in which the 
| eversion of the endocyst is perhaps in all cases complete. The endocyst 
thus constitutes a series of cells or sacs in organic continuity with each other, 
and in which the polypides surrounded by the perigastric fluid are suspended. 
fhese sacs are all closed above, where they are attached to the polypide, 
and below have in some cases their cavities in communication with those 
= 
ο΄ * Πολυποὺς, εἶδος. T Κοιψὸς, οἰκίον. 1 ᾿Εκτὸς, κύστιΞ. 
τὰ ” ᾿ ᾿ 
νεῖ Evdoy, κύστις. || Λόφος, popéw. { Περὶ, γαστήρ. 
Ἷ x2 
