RADIATA. 



699 



at once distinguished from the minute Hydra form Polypes which most resemble them, the arms of the 

 lattei being never famished with these appendages. The mouth leads to a wide funnel-shaped tube, 

 the pharynx, which soon contracts into a narrower canal or sesophagus, that terminates at its lower 

 end in the digestive cavity. The first part of this is an organ which seems closely to resemble a 

 gizzard; it is a globular form, and has two dark spots upon its sides, from which radiating lines are 

 seen ; and these seem to be composed of muscular fibres, whose office it is to effect the trituration 

 of the food, by means of teeth projecting from the inner wall of the cavity. The gizzard opens at its 

 lower end into a larger bag, which is the true digestive stomach. Its walls are thickly studded with 

 spots of a rich brown colour ; these appear to be caused by minute follicles or sacs opening from its 

 cavity and secreting bile, thus constituting the most rudimentary form of liver. From the upper part 

 of the stomach, and by the side of the entrance from the gizzard, arises the intestine, the entrance to 

 which is surrounded by vibratile cilia. This passes up as a straight tube by the side of the eso- 

 phagus, and terminates by a proper anal aperture outside the circle of tentacula. The whole of this 

 complex digeslive apparatus floats freely within the general cavity formed by the membrane that lines 

 the cell ; the intervening space being occupied by a clear fluid, and by the muscles which project the 

 animal from the cell, and which retraci it within that envelope. This space communicates with the 

 cavity of the stem, and also with the interior of the tentacula. There would appear to he no definite 

 circulating system, neither heart nor blood-vessels being discoverable. The propagation of these ani- 

 mals takes place in two ways — by gemma; or buds — and by ova or eggs. The buds are developed in 

 Bowerbankia, and in other genera whose cells arise separately from a stolon, from the stem itself; but 

 in those in which the cells are in contact with each other, and there is no common stem, as in the 

 Flustra, the cells bud off from each other. 



Now if we scrutinize these characters, we shall see that the greater number of them are rather 

 Molluscan than Zoophytic. In the first place, all the true Polypes use their arms to grasp their food and 

 to convey it to the mouth, and the arms are destitute of cilia. On the other hand, in the Ascidians 

 and all other Acephalous Mollusca, the nutritive matter is drawn in by a ciliary current, which also 

 serves to aerate the fluids. Thus, although the arms of the Bryozoa very commonly present a circular 

 arrangement, they may be considered as representing, in their relation to the economy of the animal, 

 the ciliated branchial sac of the Ascidian. But they do not by any means constantly present this radial 

 symmetry. Thus in the Plumatellu, a beautiful freshwater genus belonging to the order Hippocrepia, 

 the ciliated arms are set upon two lobes or projections, one on either side of the mouth. The structure 

 of the digestive apparatus is decidedly Molluscan. In no true Polype is there a separate intestine or 

 anal orifice ; and the existence of a gizzard-like organ, and of the rudiment of a liver (exactly resembling 

 that found in the lowest Tunieata) are also characters of elevation. The most important of all the single 

 characters furnished by the anatomy of these animals is their nervous system ; which, as already pointed 

 out, is decidedly Molluscan. The absence of a heart and distinct circulating system is, it is true, a Zoo- 

 phytic character; but we have found that, even in the true Tunieata, the circulation possessed a want 

 of constancy which indicated a tendency towards degradation. The propagation by gemmation, al- 

 though formerly supposed to be a character exclusively Zoophytic, is now known to belong also to the 

 Tunicated Mollusca; from this, therefore, no argument can be drawn in favour of the Zoophytic 

 nature of these animals. And although many of their compound fabrics have a stony density, and 

 closely resemble the solid polypidoma of the Anthozoa, yet in others, especially among the freshwater 

 species, we find a very close resemblance to the gelatinous bed or leathery orost in which the compound 

 Ascidians are lodged. And if wo imagined calcarons matter to be deposited in this bed or crust, we 

 should have a strong fabric resembling that of many Bryozoa. In their power of projecting their 

 bodies from their cells, the Bryozoa must be admitted to resemble Polypes rather than Tunieata ; but 

 this is a character of no particular importance; and •000 approaches to ii are seen among the com- 

 pound Ascidians. 



The following is the arrangement of the Bryozoa given by Dr. Johnston (British Zoophytes, m 

 edition). It must be borne in mind thai the terms polype and polypidvm an' not properly applicable to 

 these animals and their compound fabric. 



Obdeb I. — TN It NDIBULATA. 



Natives of ins sea. Polypes compound, the mouth unrounded "itb ciliated* filiform, rstraotlls u-iitui-uia, 

 which form an uninterrupted circle ; ova ciliated. 



