ORDER ACTINOIDEA. aia) 
Thus is produced the horny stem of the sea-fan (Gorgonia), which 
was long considered of vegetable origin. The polyps, which, in these 
corals, form a coating around the horny axis, often secrete, within their 
tissues, lime enough to give considerable firmness to the coating, 
making it a crust to the axis: yet in some species this crust, when 
dry, so readily peels off, that the specimens in cabinets are often the 
mere black branching axis of a Gorgonia. The coating of polyps, in 
other species, remains entirely fleshy, and in these, the axis is always 
stripped of the polyp-exterior, after drying and washing: the Anti- 
pathes are examples of this, in which the axis is corneous—and the red 
or noble coral of the Mediterranean (Corallium), an example, with a 
calcareous axis. 
The texture of these secretions in Gorgoniw, was long since well 
illustrated by Ellis; and plates ii. and ix., of his posthumous work, con- 
tain illustrations of some of the principal varieties. Sometimes they 
are solid throughout, with but slight indications of a concentric struc- 
ture. In other species, the interior is less firm, and appears like a 
pith to the stem,—a peculiarity easily accounted for by the fact that 
this portion is the production of the young or apical polyps alone 
(§§ 70, 74). Occasionally, they consist of an aggregate of short fibres, 
placed longitudinally, and compacted more or less firmly together. 
From a general survey of the facts, it is evident that these secre- 
tions are epidermic: moreover, they appear to be generally elaborated 
in short fibres, successively applied to one another, and form thus 
successive layers, which may occasionally be distinguished. They 
have a relation in origin, use, and composition, to the fibrous byssus, 
secreted by the foot of certain molluscs, and are formed like the 
epidermic appendages generally (nails, &c.) of the higher animals. 
The common Actinia is said to attach itself at base by means of a 
glutinous secretion, which may be analogous to those of the Gor- 
gonie, although differing in not admitting of accumulation. It seems 
probable, as was suggested to me by my valued associate, Dr. C. 
Pickering, that the Velella and Porpita are closely allied to the 
Actinie; and the structure of the tentacles and mouth seems to con- 
firm this opinion. If this be true, the cartilaginous shield, with the 
thin process called the sail, corresponds to the foot-secretions here 
described. The animal floats with the tentacles downward, and the 
sail raised to the wind, by which it is wafted over the surface of the 
ocean. ‘The air-disk in the base of the Actinecte, acting as a float 
for the animal, is apparently similar in origin to the disk of the 
