When the Nereis is in a state of repose, these gills are laid flat over its back; but in a state of activity they are fully spread out, and act as 'paddles,' by aid of which the animal is enabled to glide through its native element with a graceful serpentine motion.
At the base of each paddle is situated a smaller one, consisting of a fleshy pedicle shielding a fan-like bunch of hairs, each of which tapers to a sharp point. Combined, these hairs or spines form a powerful defensive weapon, which can be extended or retracted at will; and it also serves as an oar, or propeller.
As a noteworthy instance of tenacity of life in the lower animals, it may be well to mention here that I have on various occasions, by aid of the microscope, watched for several minutes the bunch of spines, above alluded to, thrust out and retracted in a single segment cut from the body of the Nereis; and only as the object became devoid of moisture did its beautiful mechanism cease to play.
The specimen now before me is comparatively small, being only twelve inches in length, yet its body contains nearly one thousand lateral appendages, constituting, it must be admitted, a most extensive and wonderful locomotive apparatus.
This Annelid is not a suitable object for the aquarium, on account of its frequent great length, and the consequent likelihood of its getting entangled among stones and rock-work when in search of food.
If the hinder parts be cut off, as has been already hinted, they will exhibit vitality for a considerable period when placed in water, but we are told it is the anterior (?) portion of the Phyllodoce which alone possesses the power of regenerating lost segments; these will be reproduced sometimes at the rate of three or four in a week.
'These creatures,' says a learned author, 'as might be expected from their activity and erratic habits, are carnivorous; and innocent and beautiful as they look, they are furnished with weapons of destruction of a unique and most curious description. The mouth of the Nereis would seem at first to be a simple opening, quite destitute of teeth; but on further examination, this aperture is found to lead into a capacious bag, the walls of which are provided with sharp, horny plates, even more terrible than those which are occasionally to be met with in the gizzards of some of the higher animals. It is not surprising, therefore, that by many anatomists the structure in question has been described as a real gizzard, or by some as the stomach itself. A little attention to the habits of the living Annelid will, however, soon reveal the true character of the organ. No sooner does the creature wish to seize its food than this so-called gizzard is at once turned inside out, in which condition it protrudes from the mouth like a great proboscis, and the teeth, which were before concealed in the interior of the cavity, now become external, and display as formidable an assortment of rasps, files, knives, saws, hooks, or crooked fangs, as any one could wish to see. Let us suppose them, when in this condition, plunged into the body of some poor helpless victim, while at the same moment the proboscis is rapidly inverted and withdrawn; the prey thus seized is at the same instant swallowed, and at once plunged into a gulf where all struggles are unavailing, there to be bruised, and crushed, and sucked at leisure.'
There is a curious fact in connection with these Annelids which is too interesting to be omitted here. I allude to the wonderful manner in which their young are produced by a process that may be called 'sprouting.'
This invariably takes place in the segment immediately preceding the terminal one. When a new animal is about to be formed, the reproductive segment swells, and after a certain time the infant worm is seen growing from the tail of its parent. When sufficiently developed, the offspring detaches itself, and starts life on its own account. Sometimes before the elder born Annelid is fully formed, the mysterious segment produces a second offspring, and, according to Professor Milne Edwards, as many as six young ones may be generated in succession from the same posterior segment, all of which will for some time continue attached to the parent worm.