ORDERACTINOIDEA. 55 



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 Gorgonise, 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 ijoung or apical polyps alone 

 (H 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 hyssus, 

 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- 

 gonise, although differing in not admitting of accumulation. It seems 

 probable, as was suggested to me by my valued associate, Dr. G. 

 Pickering, that the Velella and Porpita are closely allied to the 

 Actiniae ; 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 Actinectse, acting as a float 

 for the animal, is apparently similar in origin to the disk of the 



