EEPORT ON THE ACTINIARIA. 65 



The function of the muscuhxr bundles is easily seen ; they tend to raise the pedal 

 disk at certain points from the underlying substance, and by thus forming a vacuum, 

 cause the pedal disk to act like a sucker and secure the firm attachment of the animal. 



The wall is 1 cm. thick, and is, moreover, remarkably firm, so that it furnishes a very 

 efiiectual protection ; it feels like cartilage or like the cellulose mantle of Phallusia 

 •mammillata, and, like the latter, easily separates into shreds on division. Under the 

 microscope it shows a homogeneous fundamental substance in which fine filaments cross 

 in all directions, and form a thickly tangled layer. Each filament runs separately, and 

 can be followed some little way. From these the processes of the numerous minute cells 

 are to be distinguished by their greater thickness and fine granulation. 



The surface of the wall rises in numerous knobs 0'5 cm. across, which often have a 

 small dark spot on the highest point ; they are commonly arranged, though irregularly, 

 in transverse and longitudinal rows. The wall feels otherwise quite smooth. 



A special mesodermal circular muscle is present, even though in all the specimens the 

 oral disk was widely extended, and the waU only slightly or not at all contracted. The 

 circular muscle is of some breadth, as it measures nearly 2 cm., but its thickness can 

 hardly be measured without the microscope ; it lies close under the endoderm as a thin 

 layer of bundles of muscular fibres (PL VI. fig. 3). If we consider that the body wall of 

 the animal is not only very thick, but of cartilage-like consistency, we can easily under- 

 stand that the contraction of the muscle is unable to effect rapid closure of the oral disk. 



The bundles vary in strength, according as they consist of a smaller or greater number 

 of fine muscidar fibres; in their lower third they form a single layer, in which there is no 

 perceptible further grouping ; farther up, the bundles become arranged in rows, and then, 

 as a larger quantity of connecting substance passes in between the rows, the latter 

 radiate to the number of nine or ten into the orelatinous substance. 



The wide oral disk, whose surface is covered with indistinct radial furrows, is not so 

 strong as the wall, but, in comparison with other Actiniae, equally rich in cartilage-Kke 

 supporting substance. The radial muscles, whose bundles are compacted into a tolerably 

 thick and firm layer, lie in the oral disk, separated from the ectoderm by a broad interme- 

 diate layer of supporting substance ; some of the bundles become detached from the 

 principal mass, and run through the fundamental connective tissue towards the endoderm, 

 where they terminate. As they cross each other on the way they form an irregular 

 network. 



The tentacles are undeniably the most interesting portion of the oral disk, and 

 their odd form attracts attention even on a superficial glance. They consist of two parts, 

 a basal tuberous swelling, or bulb, and a hoUow process, or tentacle tube (PI. II. 

 fig. 9 ; PI. IX. figs. 8 and 9). The bulb is formed by a strong thickening of the support- 

 ing substance; and since this is most extensive on the peripheral side of the tentacle, 

 the canal, which is not enlarged in other respects, runs eccentrically near the adaxial side. 



(ZOOL. CHALL. EXP. PAUT XV. — 1882.) P 9 



