It is highly interesting to note the manner in which the sea-squirt obtains its food. The nutritive wherewithal consists of sedimentary matters, such as minute animals and plants, these substances being drawn into the breathing-sac along with the currents of water which are continually being taken into the body. The nutritive sediment is collected together by certain folds of the lining membrane of the breathing-chamber, and is thus transferred to the mouth-opening below. The breathing-chambers of the sea-squirts, it may be noted in passing, frequently afford lodgment to tiny dwellers in the shape of little pea-crabs. The writer has noticed these little lodgers to issue forth at night from the mouth of the sea-squirts, when the latter have been kept in an aquarium, in order to pick up particles of food on the floor of their abode. The crabs retreated to their shelter on the slightest alarm; and this case of companionship presents one of those curious instances of animal association which at present we are wholly unable to explain.
The food being converted into blood in the digestive system, we may next inquire as to the means which the sea-squirt possesses for circulating the blood through the body. In higher animals, the heart and blood-vessels perform this important task; and in the sea-squirt we find these structures to be represented; the sea-squirt’s heart indeed, in respect of its peculiarity of action, being without a parallel in the whole animal world. The heart consists of a simple tube, from each end of which blood-vessels pass, some being distributed to the breathing-chamber, and others to the body generally. In the highest animals the heart has the double function of driving pure blood through the body, and of circulating impure blood through the breathing organs for purification. It is noteworthy to observe, that by a curious and, as already remarked, altogether unparalleled contrivance, Nature has succeeded in causing the simple tube-heart of the sea-squirt to perform the work done by the complex organ of higher animals. When we observe the movements of the sea-squirt’s heart, we may see it to propel the blood by its pulsations at first to the breathing-chamber for purification. Then a pause succeeds, and the heart is observed to pulsate in the reverse direction, and to drive the blood from the breathing-chamber through the body. Probably no better illustration of the manner in which, by a modification of function, Nature compensates for simplicity of structure, could be had, than that afforded by the sea-squirt’s heart.
The breathing-chamber, as we have seen, receives fresh sea-water from the outside world, this water containing the vivifying oxygen required for the purification and renewal of the blood. Having given up its oxygen to the blood contained in the fine blood-vessels of the breathing-chamber, and its sediment to serve for food, the great bulk of water contained in the breathing-sac has now to be got rid of, to make room for a fresh supply. This process is effected in the most admirable manner through the currents created by the little filaments or cilia, which cause a constant flow of water to pass through the network walls of the breathing-chamber into a second sac or bag which lies parallel with it. This latter sac receives the name of the atrium, and communicates with the outer world by the second neck or orifice of the body. Hence the water which enters by the mouth-opening, after passing through the breathing-chamber, is ejected by the second aperture of the body, and affords the material wherewith the sea-squirt vents its indignation on prying humanity in the shape of the jets d’eau which have procured for it its popular designation. The sea-squirt regarded in relation to its sedentary habits, would seem to require no great exercise of nervous powers. Accordingly we find its nervous system to be represented by a single mass of nervous matter, placed near the mouth, and from which nerves pass to the other parts of the body. The acts of a sea-squirt may thus be regarded as purely of the character we are accustomed to name ‘automatic.’ It is provided with instincts enabling it to carry on the acts of its life and to exhibit a certain degree of irritability, without at the same time knowing the ‘reason why’ of its own actions.
The sea-squirts present no exceptions to the universal rule of Harvey, omne animal ex ovo—this philosopher believing in the universal development of the animal-form from an ovum or egg. But unlike most higher animals, the young sea-squirt does not come from the egg in the likeness of the parent. It first appears as a tadpole-like body, this creature—the larva as it is named—being produced in some thirty hours after the development of the egg begins. The head of the tadpole is provided with pigment spots or rudimentary eyes, and with three little suckers, by means of which it ultimately attaches itself to fixed bodies, prior to assuming the form of the adult and perfect Ascidian. The tail of the tadpole-larva next becomes retracted within its body, and therein disappears, whilst after fixing itself, the well-known features of the sea-squirt become duly developed. A Russian zoologist has remarked that in the tail of the sea-squirt a long rod-like body is to be seen. Now in the lowest fishes, the spine exists in a similar and rod-like condition; and hence, by a certain school of naturalists, it is urged that the vertebrates may have originated in the past from some form resembling the sea-squirt larva, in whose tail we are therefore invited to behold the first stage of the vertebrate backbone or spine. It is noteworthy to observe, however, that the opinions of these naturalists are by no means accepted by the scientific world at large; and one eminent German observer declared that the rod-like body in the sea-squirt larva’s tail was not situated in the back, but in the opposite region of the body, and that therefore it could not be regarded as corresponding to the ‘back’-bone of higher animals.
Certain near relations of the sea-squirt become of exceeding interest from their departure from the more usual and staid type of Ascidian structure. Amongst these errant members of the sea-squirt tribe the most remarkable perhaps are the Salpæ—clear, gelatinous animals, met with swimming in long connected chains on the surface of the sea in tropical regions. The celebrated novelist Chamisso, author of the charming story, Peter Schlemil or the Shadowless Man, who to his literary tastes united a striking aptitude for natural history research, discovered that the young of these chain-salpæ invariably appears as a single form; whilst each single salpa has the power of producing a connected chain. Thus the salpa sea-squirts exist in two distinct forms—chain-salpas and single salpas, and to use Chamisso’s own words: ‘A salpa mother is not like its daughter or its own mother, but resembles its grand-daughter and its grand-mother.’
Another curious group of the sea-squirts is that known by the name of the Pyrosomæ, a name literally meaning ‘fiery-bodies.’ These latter forms exist as connected masses of sea-squirts aggregated together, so as to form a hollow cylinder or tube, closed at one end; this animal-colony swimming on the surface of the sea, by the admission and forcible ejection of water from the interior of the tube. Such a means of locomotion reminds us of a veritable hydraulic engine, and is decidedly a useful modification of the common sea-squirt’s habit. The pyrosomas exhibit a strange phosphorescent light, seen also in such animals as the jelly-fishes. These luminous sea-squirts when seen in shoals, have well been described as ‘miniature pillars of fire, gleaming out of the dark sea, with an ever-waning, ever-brightening, soft, bluish light, as far as the eye could reach on every side.’ Side by side with this description from the pen of a distinguished naturalist, may be placed the poetic realisation of a similar scene by Sir Walter Scott, who in the Lord of the Isles has happily noted the luminosity of the sea when,
Awaked before the rushing prow
The mimic fires of ocean glow,
Those lightnings of the wave;
Wild sparkles crest the broken tides,