GENERAL CHARACTERS OF THE SUBKINGDOMS. Ill 



of most of the Coelenterata has hitherto escaped detection ; but in the Cteno- 

 phora or ciliograde forms, such as the Beroe, it consists of a single or compound 

 ganglion placed in the centre of that part of the body opposite to the oral aper- 

 ture, and of nervous filaments radiating from it. Doubtful ganglia have been 

 described as existing in the base or attached part of some of the Actinozoa or 

 sea-anemones. 'No organs of circulation exist ; but the cavity of the body is 

 prolonged in the form of canals, even into the tentacles, and sometimes these 

 are lined with cilia, so that they may not only convey nutriment, digested in 

 the short alimentary canal, and passed at once into the cavity of the body, but 

 may, as well as the general surface, act as respiratory organs. 



Protozoa. The remaining and lowest classes of animals constitute the sub- 

 kingdom, Protozoa, which includes the Infusoria, the Spongida or Porifera, 

 the Rhizopoda or Foraminfera, and the Gregarinida. The Protozoa agree in 

 the marked simplicity of their organization, as compared even with the lowest 

 of the Molluscoid, Annuloid, or Coelenterate animals. With the Annuloida, 

 however, some of them present a certain affinity, as shown by the class Infu- 

 soria having at one time been made to include the rotiferous or wheel-animal- 

 cules of the Annuloid subkingdom. The Protozoa have, "so far as is known, 

 no nervous system ; they have no proper alimentary canal or circulating organs ; 

 nor do any of them contain a large body-cavity, like the Coelenterata, bounded 

 by two layers, an ectoderm and an endoderm. They are composed of a minute 

 mass, or of aggregate masses, of a soft substance, usually designated sarcodous, 

 possessing more or less contractility ; within this there is often found a central 

 nucleus, and frequently one or more peculiar cavities of variable size, named 

 contractile vesicles. Nearly all inhabit either fresh or salt water, but a few live 

 in the interior of more highly organized animals. Most of them possess cilia 

 used either as locomotive organs, or for the purpose of creating currents in the 

 water in which they live. They are usually multiplied by the simplest forms 

 of development, such as gemmation or fission, as will be hereafter explained. 



Of the Protozoa, the class Infusoria certainly stand higher than the rest ; 

 their soft sarcodous substance is firmer on the surface than in the interior, 

 where it is sometimes almost fluid. In the typical forms, a small orifice on 

 the surface, surrounded by cilia, constitutes a sort of mouth ; and hence the 

 Infusoria have been grouped together under the name of Stomatoda, or Stoma- 

 tode Protozoa. This mouth leads into a shallow cavity or short tube, wnich 

 ends abruptly in the soft central sarcode, and which is regarded as a gullet^ or 

 rudiment of an alimentary canal. The contractile vesicles in their interior, 

 sometimes also named vacuoles or water-receptacles, are more numerous than in 

 other Protozoa. The Infusoria are also particularly distinguished by the im- 

 portance of their cilia, which are sometimes single, but more frequently very 

 numerous on the surface of their bodies, and which serve not only for locomo- 

 tion, but also sometimes to direct the food into their short gullet. By the 

 possession of locomotive and other cilia, they approach the Rotifera and other 

 Scolecida ; also by the analogy between their contractile vacuoles, which are 

 sometimes ramified, and the water-vessels of these Annuloida ; and, lastly, 

 by their occasionally undergoing, like the latter, a process of encysiation, pre- 

 paratory to developing young in their interior. They are distinguished from 

 the other Protozoa by the peculiar possession of the so-called nucleus and nu- 

 cleolus, two parts essentially concerned in that form of the reproductive pro- 

 cess known as conjugation. The Infusoria, like the remaining Protozoa, are 

 developed also by gemmation and by fission. 



The remaining Protozoa present no oral aperture or mouth, and hence have 

 been grouped together under the name Astomata, or Astomatode Protozoa. 

 They are, in all respects, more simple in structure than the Infusoria, their 

 sarcodous substance being destitute of any firmer outer portion or envelope, 

 but being uniformly soft throughout, and sometimes containing only a single 

 contractile water-vesicle. The Spongida consist of aggregations of these mi- 

 nute sarcodous masses, which are sometimes ciliated, and are always, as well 

 as the allied sarcodous unicellular independent animals, such as the Amreba, 

 capable of varying their form by thrusting out broad or narrow processes or 

 lobes, sometimes named feet. The Rhizopoda are furnished with beautiful 

 coriaceous or siliceous shells, sometimes simple, sometimes many-chambered : 



