474 ZOOPHYTES. 



looking residuum is left, which preserves the form of the spicule, 

 and is probably to be considered as a cell in an early stage of 

 formation, its wall not yet being differentiated as a distinct mem- 

 brane. 



310. Of the order Helianthoida, the common Actinia or "Sea- 

 Anemone" may be taken as the type ; the individual polypes of 

 all the composite structures included in the group being con- 

 structed upon the same model. In by far the larger proportion 

 of these Zoophytes, the bases of the polypes, 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 lamellse 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 intervening between the stomach and the general integu- 

 ment 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 

 polype ; on a far smaller scale, it is seen in the little Caryophyllia, 

 a like solitary polype of our own coasts, which is scarcely dis- 

 tinguishable 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 ar- 

 rangement 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 the lower 

 powers of the Compound Microscope, the former being viewed 

 by reflected and the latter by transmitted light. And thin sec- 

 tions of various fossil Corals of this group are very striking 

 objects for the lower powers of the Oxyhydrogen Microscope. 

 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 Hydraform 

 polypes has been already noticed (§ 300), 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 of 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. JSTot only 

 do these organs present difierent forms in different species ; but 



