691 



FOURTH DIVISION.— THE RADIATA. 



The group of Radiata, as left by Cuvier, is a very heterogeneous one ; for it includes, with the truly- 

 radiated animals, others which have no affinity with them. The designation is only correctly appli- 

 cable to the EcHiNODERMATA, the AcALEPH^:, and the Polypi ; all of which are characterized by a 

 more or less regular disposition of similar parts round a common centre. In the last of these classes, 

 there is a tendency to the production of compound structures, resembling those of plants, by a process 

 of gemmation or budding ; in these compound structures the radial symmetry would seem altogether 

 lost, but it is always discoverable in the individual polypes, although not exhibited by the mass. It is 

 to this group, that the term Zoophyte is properly restricted ; since it is in this alone that the plant-like 

 growth is exhibited. The Acalephte occasionally increase, like Polypes, by gemmation; but the ge"i- 

 mae become detached, and do not form a composite structure. In the Echinodermata, multiplication 

 by gemmation has not yet been observed. 



Even when thus restricted, however, the Radiated sub-kingdom will not include all the animals be- 

 longing to the Cuvierian classes of Echinodermata, Acalephae, and Polypi; for there is a large and 

 important section of the last of these divisions, which ought, as will be explained hereafter, to be 

 rather associated with the MoUusca, forming the connecting link between Tunicata and Zoophytes. 



The class of Entozoa for the most part consists of animals which should be regarded as degraded 

 forms of Articuiata ; their form, structure, mode of progression, &c., being essentially worm-like. 

 Other genera, however, especially those ranked under the family Tremadotea, would seem to be rather 

 Mollnscan in their character ; the Planarice especially approximating very closely in their form, 

 structure, and habits, to certain degraded tribes of Nudibranchiate Gasteropoda. In fact, nothing 

 but the general simplicity of organization prevalent amongst the Entozoa, and their community of 

 habitation (to which, however, the Planarice , whose habits resemble those of Leeches, constitute 

 an exception) could have caused the union into one group of forms so heterogeneous. 



The class of Infusoria is now divided into two groups, which agree in nothing but the minuteness 

 of their size, and the similarity of their habitation. The first of these, the Rotifera, ought to be 

 placed among the Articulata. The second, the Polyoastrica, must be regarded as forming the 

 lowest class of the Animal Kingdom, if, indeed, it should be admitted into it at all. As they present 

 no approach to a radiated structure, they have no title to be ranked amongst the Radiata, and must 

 form a group altogether distinct. 



CLASS ECHINODERMATA. 



The classification of this group proposed by Cuvier partook of the imperfections that necessarily 

 result from an insufficient acquaintance with the form and structure of the animals which it is desired 

 to arrange. The great increase of our knowledge in this respect has led to a much truer appreciation 

 of the value of the difl^erent groups, and of the characters according to which they should be subdivided. 

 It is remarkable, however, that notwithstanding the close gradation by which one group passes into 

 another, so that there can be no question as to their mutual affinity, it is extremely difficult, if not 

 impossible, to give any simple definition which shall include the entire class ; for even the characters 

 that are most typical of particular groups disappear entirely in others. Thus the prickles or spines 

 upon the surface, from which the class takes its name, are especially characteristic of the Echinus 

 and its allies ; they are less developed, but still recognizable, in the various Asteroid tribes; but they 

 disappear altogetlier in the Crinoidcce, which constitute the lowest order of the class, and in the 

 Uolothuridee and Sipunculidee , which in many points of their organization are the highest. Again, 

 the skeleton, whose peculiar structure will be presently described, is fully developed in the Crinoidece, 

 in the Echinus, and in Star-fish; but nothing more than a mere rudiment of it exists in the Jloluthu- 

 ridce, and it disappears altogether in the Sipxmcididce . The most universal character, perhaps, is the 



