ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 355 
It is rare for any other form of zooid besides the polypite to enter into the 
composition of the trophosome. In a few genera, however (Plumularia and 
its allies), we find associated with the polypites certain very remarkable zooids, 
whose function, though obscure, is more likely to be connected with the nu- 
trition of the colony than with anything else. The bodies in question have 
been named “ nematophores” by Busk*. They are situated upon certain 
definite points of the trophosome, and consist of minute tubular receptacles 
containing a soft granular protoplasm, and frequently, though not always, a 
cluster of large thread-cells. The protoplasm which fills these receptacles 
can spontaneously emit prolongations of its mass in no way distinguishable 
from the. pseudopodia” emitted by the true Rurzopopa. In fact, the con- 
tents of the nematophores present in this respect no appreciable difference 
from the protoplasm composing the body of an Ameba. The thread-cells, 
however, though apparently immersed in the protoplasm, do not seem to be 
ever carried out in the pseudopodial prolongations, and it is probable that the 
portion of the contents of the nematophore with which they are in immediate 
connexion does not participate in the power of emitting pseudopodia. 
The trophosome may consist of only a single polypite (Corymorpha), or, 
what is by far the most frequent condition, it may consist of many associated 
into a compound group or colony. All the zooids of a colony, both those be- 
longing to the trophosome and those belonging to the gonosome, are kept in 
organic union with one another by a common connecting tubular basis (fig. 1 
d,e). To this common basis of communication I have elsewheret given the 
name of “ ccenosare.” 
Both zooids and ccenosare are composed of two membranes, an outer or 
“ectoderm” and an inner or “ endoderm,” the ectoderm having its free sur- 
face in direct relation with the outer world, while the free surface of the 
endoderm is turned inwards and forms the boundary of the gastric cavity 
and of all its prolongations through the organism+. A similar composition 
Tay be demonstrated not only in all the rest of the Hyprozoa, but in the 
Whole group of the Carenrerata. For the important generalization which 
thus asserts the composition of every ccelenterate animal out of two mem- 
branes—a generalization which forms the basis of the whole morphology of 
the Coelenterata—we are indebted to Professor Huxley, who first enunciated it 
a8 a great anatomical truth §. 
The ectoderm invariably contains imbedded in it the peculiar bodies known 
as thread-cells, which are frequently aggregated in definite groups, very cha- 
acteristic of the species in which they occur. A fibrillated contractile tissue, 
esembling non-striated muscular fibre, may also in a great many instances 
@ demonstrated in connexion with the ectoderm. 
Tn every member of the Hyprorpa with whose trophosome we are ac- 
uainted, excepting only the freshwater Hydra and probably also Nemopsis, 
organs, which occur at a distance from the polypites, and arise from the common con- 
ecting basis of the colony. y 
* Busk, MS. Lectures on Comparative Anatomy delivered in the Royal College of 
urgeons, London. 
t “On the Anatomy and Physiology of Cordylophora,” Phil. Trans. 1853. 
Tt From this generalization, however, we must except the “ Nematophores,” in whose 
aple granular ameboid protoplasm no differentiation into ectoderm and endoderm can be 
§ See his Memoir on the Anatomy and Affinities of the Meduse, Phil. Trans., June 1849 ; 
}s0 his ‘ Oceanic Hydrozoa,’ published by the Ray Society, 1859, and Lectures on General 
at, Hist., in the ‘Medical Times.’ 
242 
