SPONGES AND ZOOPHYTES. 135 



527. Of the Zoantharia, the common Actinia or ' sea anemone ' may 

 be taken as the type; the individual polypites of all the composite fabrics 

 included in the group being constructed upon the same model. In by 

 far the larger proportion of these Zoophytes, the bases of the polypites, 

 as well as the soft flesh that connects together the members of aggregate 

 masses, are consolidated by calcareous deposit into stony Corals; and the 

 surfaces of these are beset with ' cells,' usually of a nearly circular form, 

 each having numerous vertical plates or lamellcs radiating from its centre 

 towards its circumference, which are formed by the consolidation of the 

 lower portions of the radiating partitions that divide the space interven- 

 ing between the stomach and the general integument of the animal into 

 separate chambers. This arrangement is seen on a large scale in the 

 Fungia or ' mushroom-coral ' of tropical seas, which is the stony base of 

 a solitary Anemone-like animal; on a far smaller scale, it is seen in the 

 little Caryophyllia, a like solitary Anemone of our own coasts, which is 

 scarcely distinguishable from an Actinia by any other character than the 

 presence of this disk, and also on the surface of many of those stony 

 corals known as ' madrepores;' whilst in some of these the individual 

 polype-cells are so small, that the lamellated arrangement can only be 

 made-out when they are considerably magnified. Portions of the surface 

 of such Corals, or sections taken at a small depth, are very beautiful 

 objects for low powers, the former being viewed by reflected, and the 

 latter by transmitted light. And thin sections of various fossil Corals of 

 this group are very striking objects for the lower powers of the Oxy- 

 hydrogen Microscope. 



528. The chief point of interest to the Microscopist, however, in the 

 structure of these animals, lies in the extraordinary abundance and high 

 development of those ' filiferous capsules,' or ' thread-cells,' the presence 

 of which on the tentacles of the Hydroid polypes has been already 

 noticed ( 514), and which are also to be found, sometimes sparingly, 

 sometimes very abundantly, in the tentacles surrounding the mouth of 

 the Medusae, as well as on other parts of their bodies. If a tentacle 

 of any of the Sea-anemonies so abundant on our coasts (the smaller and 

 more transparent kinds being selected in preference) be cut-off, and be 

 subjected to gentle pressure between the two glasses of the Aquatic-box 

 or the Compressorium, multitudes of little dart-like organs will be seen 

 to project themselves from its surface near its tip; and if the pressure be 

 gradually augmented, many additional darts will every moment come into 

 view. Not only do these organs present different forms in different 

 species, but even in one and the same individual very strongly marked 

 diversities are shown, of which a few examples are given in Fig. 361. 

 At A, B, c, D, is shown the appearance of the 'filiferous capsules,' whilst 

 as yet the thread lies coiled-up in their interior; and at E, F, G, H, are 

 seen a few of the most striking forms which they exhibit when the thread 

 or dart has started-f orth. These thread-cells are found not merely in the 

 tentacles and other parts of the external integument of Actinozoa, but 

 also in the long filaments which lie in coils within the chambers that 

 surround the stomach, in contact with the sexual organs which are 

 attached to the lamellae dividing the chambers. The latter sometimes 

 contain ' sperm-cells ' and sometimes ova, the two sexes being here 

 divided, not united in the same individual. What can be the office of 

 the filiferous filaments thus contained in the interior of the body, it is 

 difficult to guess-at. They are often found to protrude from rents in the 

 external tegument, when any violence has been used in detaching the 



