The arms of this hydra are rows of short hairs, the body trumpet-shaped.
This species of hydra is very common, and has been described by almost every writer on these subjects; it is placed by Müller among the vorticellæ.
Vorticella stentorea caudata, elongata, tubæformis limbo ciliato. Müller animalcula infusoria.
Mr. Baker originally named it the funnel-like polype, which Messrs. Trembley and Reaumur changed to the tunnel-like polype, under which name it appears in the Philosophical Transactions, No. 474.
There are three kinds of them, which are of different colours, green, blue, and white. The white ones are the most common. It is necessary to observe them often, and in various attitudes, in order to obtain a tolerable idea of their structure. They do not form clusters, but adhere singly by their tail to whatever comes in their way; their anterior end is wider than the posterior, and being round, gives the animal somewhat of a funnel form, though it is not completely circular, having a sort of slit or gap that interrupts the circle. The edge of this opening is furnished with a great number of fibrillæ, which by their brisk and continual motions excite a current of water; the small bodies that float or swim near this current, are forced by it into the mouth of the little animal. Trembley says, that he has often seen a number of very small animalcula fall one after another into the mouth, some of which were afterwards let out again at another opening, which he was not able to describe.
They can fashion their mouths into several different forms. If any thing touch them, they shrink back and contract themselves. They live independent of each other, swimming freely through the water in search of their prey, and fix to any thing they meet with.
These animals multiply by dividing themselves, not longitudinally, nor transversely, but sloping and diagonal wise; the proceedings in nature continually varying in every new form of life. Of the two polypes produced by the division of one, the first has the old head and a new tail; the other, the old tail and a new head.
To make the description more clear, Trembley called that with the old head the superior polype, that with the new head the inferior one. The first particular that is observable in these polypes, when they are going to divide, is the lips of the inferior one; a transverse and oblique stripe indicates the part where it is going to divide; the new lips are formed at about two-thirds of the length of the polype, reckoning from the head; the division is made in a sloping line, that goes about half way round the parent animal; these lips are at first discerned by a slow motion, which engages the attention of the observer. They then insensibly approach each other and close, whereby a swelling is formed on the side of the polype, which is soon found to be a new head. When the swelling is considerably increased, the two polypes may be plainly distinguished. The superior one being now connected with the inferior one only by its lower extremity, is soon detached from it, and swims away to fix itself on some convenient substance; the inferior one remains fastened to the place where the original polype was fixed before the division.
From the various modes by which different species of polypes are multiplied, we are led to form more exalted ideas of nature, and to see that the little we discover is but an exceeding small part of her contents; we learn also to be more cautious in reasoning from analogy, and laying down the known for a model to the unknown, because we find that the operations in nature are varied ad infinitum.
The growth of the hydra fusca is very quick, but that of the hydra stentorea is much more so. The progress of the fœtus is always more rapid than that of the infant and adult animal; but in these organized atoms the evolution is so rapid, as to appear almost like an immediate creation.