698 



RADIATA. 



CLASS POLYPI. 



Of this class, now more commonly termed Polypifera, murh more is now known than was known 

 to Cuvier ; ami his classification has altogether given place to one in which the primary characters are 

 drawn from the structure of the animal, that derived from the compound mass or polypidom being of 

 secondary importance. The separation of the Carnosi or Actiniform polypes from the Gelatinosi or 

 Hydrafurm polypes, was, as we shall see, quite correct; but, on the other hand, the separation of 

 the solitary genera from the compound forms of the same groups was altogether erroneous. Thus 

 among the Coralliferi of Cuvier, the greater part of the first family is composed of Hydraform 

 polypes ; the second is made up of one genus (Cellularia) which is not a Zoophyte at all, but a Mollus- 

 can, and of another (Corallina) which is now well ascertained to be of vegetable character; whilst 

 the third is a heterogenous assemblage of .Molluscans, with Actiniform and Alcyonian Polypes, and 

 Sponges. 



The primary division of the Cuvierian Polypifera is now generally admitted to be into Bryozoa and 

 Anthozoa ; the former being truly Molluscans, and properly forming part of the class Tunicata; 

 whilst the latter are true Radiated animals. To the latter alone, therefore, ought the name of Zoo- 

 phytes to be restricted. An outline view of the structure and classification of each group will now 

 be given. 



BRYOZOA. 



If we imagine the minute tentacula which fringe the oral orifice of many Ascidians to be greatly 

 prolonged and clothed with cilia, whilst on the other hand, the respiratory chamber or dilated pharynx 

 is contracted, we shall have the likeness in its most important characters, of the animal of the Flustra, 

 Bowerbankia, or any other Bryozoon It is not surprising that, until the structure of these animals 

 had been investigated, the stony and horny fabrics which they form should have been regarded as poly 

 pidoms. And even since the wide differences in conformation between the Bryozoa and the Anthozoa 

 have been made known, the former as well as the latter have been frequently ranked among the Poly 

 pifera. The discovery of the gemmiparous development of the true Tunicata, however, has removed 

 one of the great boundaries that seemed to divide them from the Bryozoa; whilst, 

 on the other hand, the existence of forms among the latter that present a very 

 near approach to the former, and more especially the discovery that their nervous 

 system is not formed upon the radiated type, but consists of a single ganglion 

 placed between the two orifices, as in the Tunicata, have led to their entire 

 detachment from the class Polypifera, and their removal to the Molluscan series. 

 The grounds of this separation, and the relations of the Bryozoa to the 

 Tunicata and Polypifera respectively, will be better understood when the 

 structure of the animals has been examined. To this, therefore, we now 

 proceed, taking as our type a very common British species, the Bowerbankia 

 densa, in which, from the isolation and transparency of the shell or sheath, 

 the internal arrangement can be very distinctly seen. The animal of the 

 Bowerbankia, when the tentacula are fully expanded, is about half an inch in 

 length, and the cell does not nearly extend to the base of the tentacula ; the 

 animal can be retracted, however, so as to be completely protected by the cell, 

 the edges of which are drawn in so as to close the aperture. The cells of the 

 Bowerbankia are horny in their texture, and arise separately from a sort of 

 stolon or creeping stem, very much after the manner of the separate individuals 

 of the Porophora (see Appendix to Mollusca, Fig. 7): in many other genera, 

 however, a solid calcareous fabric is produced, in which the cells are imbedded ; 

 whilst in other instances, again, this fabric is soft in its texture, being sometimes 

 gelatinous as in the compound Ascidians, in other instances spongy as in the 

 Alcyonian Polypes. The tentacula, of which there are ten in the Bowerbankia, but a greater 

 number in many other genera, are always furnished with cilia ; by this character these animals are 



Fig. 10.— Bowerbankia 

 o, oesophagus ; b, gizzard 

 c, stomach ; d, orifice of in 

 lescine. 



