MR. R. OWEN’S REMARKS ON THE ENTOZOA. 391 
system obtains is so great, that it may still be regarded, with little inconvenience, as the 
chief character of the Acrita. 
I have already observed that the absence of an egestive outlet to the alimentary canal 
cannot be assigned to the Acrita as a character of that division ; but there is a condition 
of the digestive system in relation to the parietes or substance of the body, which is as 
generally applicable to the Acrita as the molecular distribution of the nervous sub- 
stance ; in this division the intestine is not separated from the skin by an abdominal 
cavity, but, whatever be its form, is essentially a simple excavation of the parenchyma. 
Now the few genera which recede from this character are precisely those in which the 
existence of nervous filaments is perhaps least ambiguous, as, for example, in Actinia 
and Beroé. It is fortunate for the systematic naturalist that these genera form so 
small an exception to the rule in regard to the conditions of the nervous and digestive 
systems in the Acrita, since the filamentary disposition of the nervous substance, in co- 
existence with a distinct abdominal cavity and muscular parietes of the alimentary canal, 
constitute the distinctive character of the remaining Radiated classes from which the 
Acrita are separated. 
In all the Acrite classes, then, we find that there is an internal digestive cavity, one 
of the chief characteristics of the animal kingdom. In the Sterelmintha, as in the ma- 
jority of the Acrita, there is but a single communication with the exterior of the body, 
with the apparent exception in the genus Cenurus, in which, as in the composite Zoo- 
phytes, nutrition is effected by numerous mouths, but without an anus. 
The vascular system, where traces of it are met with in the Acrita, corresponds with 
the digestive system, being equally devoid of proper parietes, and consisting of reticu- 
late canals excavated in the parenchymatous substance of the body, generally super- 
ficial, and in which a cyclosis of the nutrient fluids is observed analogous to that of 
plants ; but there is no true circulation. This condition is met with as low down in 
the scale as the Polygastrica, where Professor Ehrenberg has determined the existence 
of a superficial network of hyaline canals. In those genera of the Sterelmintha which 
manifest traces of a sanguiferous system, the fluids undulate in canals of a similar struc- 
ture, form, and position, as in the Trematoda, especially the Planarie, and in Echino- 
rhynchi, in some of the species of which latter the cutaneous vascular network is ex- 
tremely rich!. 
In the Meduse, among the Acalephe, the condition of the vascular system is equally 
simple with that of the lowest Acrita, as is exemplified in the marginal vascular reticu- 
is connected with two filaments that decussate one another at about the middle of their course. These he de- 
scribes as forming part of a nervous circle placed, throughout the greater part of its course, immediately along 
the bases of the row of tentacles that surround the disc, so as to form, as it were, the outer wall of the circular 
vessel, or appendage to the digestive cavity which runs round the margin of the disc. Ehrenberg further de- 
scribes another nervous circle, composed of four ganglion-like masses, disposed around the mouth, each being 
in connexion with a corresponding group of tentacles.—Miiller’s Archiv., 1834, p. 662. 
* Echinorhynchus vasculosus, Rud., Syn., p. 581. 
3 F2 
