This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(165). It is in seizing and devouring their prey, however, that the habits of the Actiniae are best exemplified. They will remain for hours with their arms fully expanded and motionless, waiting for any passing animal that chance may place at their disposal, and when the opportunity arrives are not a little remarkable for their voracity and for their capability of destroying their victims. Their food generally consists of crabs or shell-fish, animals apparently far superior to themselves in strength and activity; but even these are easily overpowered by the sluggish yet persevering grasp of their assailant. No sooner are the tentacles touched by a passing animal than it is seized, and held with unfailing pertinacity; the arms gradually close around it; the mouth, placed in the centre of the disk, expands to an extraordinary size; and the creature is soon engulfed in the digestive bag of the Actinia, where the solution of all its soft parts is rapidly effected, the hard undigestible remnants being subsequently cast out at the same orifice.
(166). The Actiniae possess the power of changing their position: they often elongate their bodies, and, remaining fixed by the base, stretch from side to side, as if seeking food at a distance: they can even change their place by gliding upon the disk that supports them, or detaching themselves entirely, and swelling themselves with water; they become nearly of the same specific gravity as the element they inhabit, and the least agitation is sufficient to drive them elsewhere. When they wish to fix themselves, they expel the water from their distended body, and, sinking to the bottom, attach themselves again by the disk at their base, which forms a powerful sucker.
(167). From the above sketch of the outward form and general habits of these polyps, the reader will be prepared to examine their internal economy and the more minute details of their structure. On examining attentively the external surface of the body, it is seen to be covered with a thick mucous layer resembling a soft epidermis, which, extending over the tentacula, and the fold around the aperture of the mouth, is found to coat the surface of the stomach itself: this epidermic secretion forms, in fact, a deciduous tunic, that the creature can throw off at intervals. On removing this, the substance of the animal is found to be made up of fasciculi of muscular fibres, some running perpendicularly upwards towards the tentacula, while others, which cross the former at right angles, pass transversely round the body; the meshes formed by this interlacement are occupied by a multitude of granules, apparently of a glandular nature, giving the integument a tuberculated aspect: these granules are not seen upon the sucking-disk at the base. The tentacula are hollow tubes, composed of fibres of the same description.
The stomach is a delicate folded membrane, forming a simple bag within the body; it seems to be merely an extension of the external tegument, somewhat modified in texture.
(168). On making a section of the animal, as represented in fig. 36, the arrangement of these parts is distinctly seen, a being the muscular integument; b, the tentacula, formed by the same fibrous membrane; and c, the stomach. Between the digestive sac, c, and the fibrous exterior of the body, a, is a considerable space, d, divided, by a great number of perpendicular fibrous partitions, I, into numerous compartments, which, however, communicate freely with each other and likewise with the interior of the tentacula, as seen at e. Every tentacle is perforated at its extremity by a minute aperture, b, whereby the sea-water is freely admitted into these compartments, so as to bathe the interior of the body; and when, from alarm, the animal contracts itself, the water so admitted is forcibly expelled in fine jets through the holes by which it entered. There can be no doubt that the surrounding fluid, thus copiously taken into the body, is the medium by which respiration is effected; and every one who has been in the habit of keeping Actiniae in glass-vessels for the purpose of watching their proceedings must have noticed that, as the fluid in which they are confined becomes less respirable, from the deficiency of air, the quantity imbibed is enormous, stretching the animal until it rather resembles an inflated bladder than its original shape.
(169). It is in the compartments thus (at the will of the creature) distended with water that we find the organs of reproduction, which here assume a development far exceeding what we have noticed in other zoophytes. On raising a portion of the membrane forming the stomach, as at f, we see lodged in each partition an immense number of granular corpuscles attached to a delicate transparent membrane, and arranged in large clusters, g. The ovigerous membrane that secretes these corpuscles is represented unravelled at h; it is through its whole extent bathed with water admitted into the compartment wherein it is lodged - a circumstance which provides for the respiration of the embryos during their development.
(170). We learn from the researches of MM. Kblliker and Erdl that the Actiniae are bisexual, and that the male and female organs are allotted to different individuals, the testes of the male and the ovaria of the female being so similar in their structure and appearance that the difference between them is only appreciable with the microscope.
Fig. 36. Section of Actinia.
In both sexes the reproductive apparatus consists of riband-like convolutions attached by delicate membranous folds to the free margins of the septa, and filled with multitudes of the granular-looking bodies above mentioned, which in the females are the ova, in the males the spermatic capsules. In neither sex is there any excretory duct; so that the eggs, when mature, must escape immediately into the general cavity of the body by bursting through the delicate membranous envelope in which they are enclosed. The whole exterior of the organ is densely ciliated.