48 



ACRITA. 



genera of sterelraintha or parenchymatous in- 

 testinal worms which manifest traces of the 

 circulating system, the fluids undulate in 

 canals of a similar structure, as is displayed in 

 the planariae, and parasitic trematoda, and also 

 in the echinorhynchi, in some species of which 

 genus the cutaneous canals form a rich net- 

 work.* In the acalephae the condition of the 

 vascular system is equally simple with that of 

 the lowest Acrita, as is exemplified in the mar- 

 ginal reticulate canals in the disk of the rhizos- 

 toma. In the Nematoneura, on the contrary, 

 those classes which manifest a circulating sys- 

 tem distinct from the digestive tube, as the 

 echinoderma and rotifera, possess vessels with 

 proper parietes, distinguishable into arteries 

 and veins. 



No Nematoneurous class presents an example 

 of generation by spontaneous fision or gem- 

 mation, but these modes of reproduction are 

 common in the Acrite division. 



The planariae among the sterelmintha are 

 capable of indefinite multiplication by simple 

 division; and the medusae are stated to pro- 

 duce, not ova, but ciliated locomotive gem- 

 mules or internal buds. The various examples 

 of these plant-like modes of generation which 

 the polypi and polygastrica present are fa- 

 miliar to most persons, and will be especially 

 treated of under their respective articles. 



The fissiparous and gemmiparous modes of 

 reproduction are not, however, the exclusive 

 modes by which the Acrite classes are perpetu- 

 ated. Most of the sterelmintha are propagated 

 by means of ova : in the cystica and cestoi- 

 dea, the generative organs consist of ovaries 

 alone, or are cryptandrous ; in the tremato 

 da, a fecundating gland is superadded to the 

 ovary; while in the acanthocephala the sexes 

 are separate, so that thus early in the animal 

 kingdom, we find typified all the different 

 modes of generation by which the race is con- 

 tinued in the higher classes of animals. 



The different conditions of the important 

 organic systems which are thus seen to obtain 

 in the great group of animals called Radiata 

 and Zoophyta fully justify a partition of 

 the group corresponding with those differ- 

 ences. For the lower organized division we 

 retain the name proposed by Macleay, but ex- 

 tend its application to the acalephae ; and thus 

 constituted it may be characterized as follows. 



Sub-kingdom ACRITA. Gelatinous polymor- 

 phous animals, without distinct nervous fibre, 

 or visceral cavities. 



Alimentary canal excavated in the parenchyma 

 of the body, generally without an anus. 



SanguiJ'erous system composed of reticulate 

 canals without proper tunics. 



Generation in most fissiparous or gemmi- 

 parous ; in some oviparous.-^ 



The Acrita have been termed Protozoa, as 



* Rudolph! terms one species echinorhynchus 

 vasculosus, from this circumstance. Synopsis Ento- 

 zoonun, p. 581. 



t The definition of the Acrita given by Macleay 

 is confessedly a negative one as referred to animals j 

 it is as follows : 



being on the first step of animal organization. 

 They are analogous to the ova or germs of the 

 higher classes, and have, therefore, been termed 

 by Carus Oozoa ; and as the changes of the 

 embryo succeed each other with a rapidity 

 proportionate to the proximity of the ovum to 

 the commencement of its development, so also 

 we find that in each class of Acrita there are 

 genera which advance into close approximation 

 with some one or other of the classes belong- 

 ing to the higher divisions of the animal king- 

 dom. It results, therefore, from this tendency 

 to ascend in the scale of organization that there 

 is greater difficulty in assigning constant or gene- 

 ral organic characters to the Acrita than to any 

 of the higher divisions of animals. Even in 

 the nervous system, we find as we are led step 

 by step from the hydra to the actinia in 

 the class Polypi, that the nervous globules 

 begin to manifest the filamentary arrangement 

 about the oral orifice in the last named genus. 

 That, again, in tracing the successive complica- 

 tion of the sterelmintha from the hydatid to the 

 echinorhynchus we also come to perceive traces 

 of longitudinal nervous filaments in the latter 

 highly organized genus of parenchymatous 

 worms. In the acalephae the examples of the ag- 

 gregate form of the nervous system would seem 

 to be more numerous and distinct. Ehrenberg 

 has detected what he considers as a nervous sys- 

 tem in a gelatinous medusa; and Dr. Grant 

 has recently described a nervous collar giving 

 off simple filaments in the more highly or- 

 ganized beroe, which, in its distinct intestine 

 and anal outlet, recedes too far from the medu- 

 sidae to be placed in a natural arrangement in 

 the same class. Many of the polygastrica are 

 endowed with simple visual organs or ocelli, 

 in the form of red or yellow spots; similar 

 organs of a dark colour are exhibited by the 

 planariae, and Nordmann also describes them 

 in some internal parasitic trematoda. Ehren- 

 berg has recently discovered coloured ocelli 

 in a medusa, and he ascribes a sense of 

 taste to the polygastrica. 



The indications, however, of the special senses 

 in the Acrita are feeble and obscure, and in the 

 least doubtful instances the organs are evidently 

 of the simplest and most elementary nature. 



For the most part all the different systems 

 seem blended together, and the homogeneous 

 granular parenchyma possesses many functions 

 in common. 



Where a distinct organ is eliminated it is often 

 repeated indefinitely in the same individual. 

 Thus in the polypi the nutritious tubes of one 

 individual are generally supplied by numerous 

 mouths, and it has, consequently, the semblance 



tf Animalia gelatinosa polymorpha, interaneis 

 nullis medullaque indistincta. 



" Os interdum indistinctum, sed nutritio absorp- 

 tioue externa vel interna semper sistit. Anus 

 nullus. 



" Reproductio fissipara vel gemmipara, gemmis 

 modo exteris, modo internis, interdum acervatis. 



" Pleraque ex individuis pluribus semper cohae- 

 rentibus animalia composita sistunt." Horae Ento- 

 mologicae, ii. p. 224. See also Lamarck, Anim. 

 saas Vertebres, ii. p. 2. 



