356 REPORT— 1863. 
with its curious free trophosome, described by McCrady*, and Acaulis, another 
genus with a free trophosome, described by Stimpson, the ectoderm excretes 
from its outer surface an unorganized pellicle chemically identical with chi- 
tine, and forming an external tubular investment for the soft organized ecto- 
derm. The extent to which the ectoderm is covered by it varies: in some 
cases it is confined to the coonosarc; in others it extends not only over the 
entire ccenosarc, but is often continued for a greater or less extent, and in a 
more or less modified form, over the various zooids of the colony. In the 
Sertularida and Campanularida (fig. 1) it forms cup-like receptacles—the 
“ hydrothece”’ of Huxley, into which the polypites are retractile. It is in- 
variably absent from those zooids which have detached themselves from the 
colony, in order to lead an independent life in the open seat. For this un- 
organized excretion, which must be placed in a totally different category 
from that of the ectoderm and endoderm, I propose the name of periderm§. 
Two different classes of zooids may combine to make up the gonosome||. To the 
more important and only constant one of _. = 
these classes (fig. 2 ¢) I have elsewhere J Fig. 2.—Group consisting of three 
given the name of gonophore, a term zooids from a colony of Hydrac- 
which I shall continue to employ in the tinia echinata, taken from near 
present Report. The gonophore is either the margin of the colony. 
the ultimate generative zoord, giving origin 
directly to the generative elements, or 
it gives origin to these elements through 
the medium of a special sexual bud which 
is developed from it. Though possessing 
intimate homological relations with the 
polypite, it is nevertheless constructed 
upon a special type, and may be in every 
case referred to the plan, frequently much 
modified, of the so-called ‘‘ gymnoph- 
thalmic medusa.” It often separates 
itself from the rest of the hydroid, and 
then lives as a free locomotive zooid in 
the open sea (fig. 1 9). 
The other class (fig. 2b) is not neces- 
sarily present. It consists of peculiarly 
modified polypites, having their aliment- 
ary function more or less suppressed, but 
never detaching themselves from the tro- 
phosome so as to enjoy an independent 
existence. They are the “ gonoblastidia” 
of Huxley. : a, Alimentary polypite; 66, gono- 
The gonophore is always borne as @  pjastidium beara Z The gonoghames 
bud, either directly upon some part of d, spiral polypite, developed close to the 
the trophosome, or upon the gonoblasti- margin of thecolony. The three zooids 
dium, or upon another gonophore which are connected to one another by a com- 
is not then the ultimate generative zooid. ™0n basal expansion or comnosare, 
* “ Gymnophthalmata of Charleston Harbour,” in Proc. Elliott Society of Nat. Hist. of 
Charleston, South Carolina, 1859. 
+ ‘Synopsis of the Marine Invertebrata of Grand Manan,” in Smithsonian Inst. vi. 1854. 
{ Itisalso entirely absent fromthe SipHonorHOoRA. § epi, around, and dépya, skin. 
|| Without including the peculiar receptacles in Plumularia and its allies, described 
below under the name of “ corbule.” ¥ Proc. Roy. Soc. Edinb. Session 1857-58. 
