MR. R. OWEN’S REMARKS ON THE ENTOZOA., 389 
deed, merged into the class Annelida, as Rudolphi proposes, but are made a distinct 
class in conjunction with the Epizoaria of Lamarck, which Mr, MacLeay imagines may 
be entitled to a place between the Anoplura of Dr. Leach and the Chilognatha. 
The simple filamentous disposition of the nervous system, however, which opposes 
Rudolphi’s views of the place which the Nematoidea ought to hold, equally forbids their 
allocation as an annectant of any of the classes of the Annulose or Homogangliate divi- 
sion of the animal kingdom. 
The Acrita of MacLeay are thus defined. 
‘Animalia gelatinosa polymorpha, interaneis nullis, medullaque indistincta, 
‘Os interdum indistinctum, sed nutritio absorptione externa vel interna semper 
sistit. Anus nullus. 
‘* Reproductio fissipara vel gemmipara, gemmis modo externis modo internis, inter- 
dum acervatis. 
“ Pleraque ex individuis semper coherentibus animalia composita sistunt.” 
This definition, able, comprehensive, and accurate as it undoubtedly was, agreeably 
to the state of anatomical and zoological knowledge at the time when it was penned, 
fails now to convey a just idea of the prevailing and characteristic organical conditions 
of the Acrita. Fourteen years of subsequent research, crowned by discoveries of which 
the most brilliant relate to the structure and economy of the lowest classes of the 
animal kingdom, have wrought their wonted effect on what was intended at a pre- 
vious period to be the most general expression of the existing knowledge of the orga- 
nization of a natural group, ; 
The discoveries of Professor Ehrenberg with reference to the digestive system of the 
Monads not only obviate the necessity of ascribing a mode of nutrition by external ab- 
sorption to any, even the lowest of the animal kingdom ; but, while they prove the ex- 
istence of a complicated internal digestive cavity in the Agastria of M. De Blainville, 
show also that in many of the genera of this simple class the alimentary cavity is pro- 
vided with a distinct anal outlet. The larger fecal pores of the Spongie may be also 
considered in the same relation to the digestive system. 
With respect to generation, the organs of which function afford in their varieties the 
least certain indications of the relative perfection of the species, it may be observed that 
in the Twni@ the race is propagated neither by spontaneous fission nor gemmation, but 
by true ova, frequently formed by, and contained in, distinct ovaries, of which one is 
appropriated to each joint. The ova in these receptacles are commonly advanced in 
proportion as the joints recede from the head; and although the ovaries have distinct 
outlets either at the middle part or margin of the segments, yet these are commonly 
detached as the ova in the ovary are matured, in a manner analogous to the bursting of 
the external ovisacs of the Lernee and Monoculi. In another order of Parenchymatous 
Worms, the Trematoda, distinct fecundating glands are superadded to the productive 
or female apparatus, and some physiologists suppose, with Cuvier, that generation is 
VOL. I. 3F 
