314 ‘ REPORT—1850. 
of permitting a transudation of fluid. Rapp detected numerous minute 
apertures in the external walls of Actinia, keeping up a communication be- 
tween the interior of the animal and the surrounding water; the discovery 
of Rapp I have fully confirmed, and yet the apertures are so small as to 
render it certain that they would have remained undiscovered were it not 
that their presence is betrayed by a minute stream which escapes from 
them during the contraction of the animal, which occurs immediately on 
its being removed from the water. May not similar apertures exist in the 
Polyzoa? and if so, I should feel inclined to seek them in the walls of the 
alimentary canal, perhaps of the rectum. The fluid which circulates in the 
perigastric space is not perfectly homogeneous, and numerous corpuscles of 
very various and irregular shape may be observed to float through it and be 
carried about by its current. Some of these corpuscles are perhaps sperma- 
tozoa; others are of no definite shape, and look like minute portions of the 
tissues separated by laceration. May they not be some of the products of 
digestion which have transuded through the walls of the alimentary canal, 
being thus conveyed into the only representative of a true circulation with 
which these animals present us? 
The true signification of the perigastric fluid is a point whose determina- 
tion must be of great importance in the physiology of the Polyzoa. If it be 
admitted, as I think it must be, that it consists mainly of water which has 
obtained entrance from without, it then corresponds to a true aquiferous 
system subservient to a respiratory function. But, as we have already seen, 
it is probable that it receives certain products of digestion which had trans- 
uded through the walls of the alimentary canal; it thus connects itself 
with the digestive system. It is moreover the only representative in these 
animals of a sanguiferous circulation, for in the Polyzoa there is certainly 
no trace of a heart, nor can anything referable to a true vascular system be 
detected. ‘The perigastric circulation therefore unites in itself the triple 
function of a chyliferous, sanguiferous and respiratory system. 
The next point of interest to determine, with regard to the perigastric fluid, 
is the cause of the peculiar currents observed in it. These currents, which ex- 
tend into the tentacular crown, were long ago observed by Trembley* in Lopho- 
pus erystallinus ; but this author contented himself with simply recording their 
existence, and made no attempt to explain them. Nordmann‘, who observed 
them in both freshwater and marine genera, not being able to detect any 
trace of cilia or other moving power, compared them to the currents in the 
cells of Chara. That they are” produced by the action of vibratile cilia, 
there can, however, now be no doubt. Van Benedenf, tells us that he has 
seen these cilia, not only on the walls of the perigastric space, but on the ex- 
ternal surface of the alimentary canal. I cannet, however, confirm their 
existence in the latter situation; indeed, my own observations are entirely 
opposed to their presence on the alimentary canal; and I cannot help 
thinking that this statement of Van Beneden is connected with some error 
of observation. I have, however, most distinctly seen them on the upper 
part of the tentacular sheath in Plwmatella during the exserted state of the 
polypide; on other parts of the endocyst I have not succeeded in detecting 
them by direct observation; but the peculiar acceleration which the motion 
of the circulating corpuscles experiences when these approach the walls of 
the perigastric space, plainly indicate the presence of vibratile cilia in this 
situation. 
ι D. Muscular System.—The muscular system is highly developed; we 
* Loc. cit. + Micrographische Beitrage, Βα, ii. p. 75. 
+ Quelques Observations sur les Polypes d’eau douce, loc. cit, 
+ 
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