Page:Popular Science Monthly Volume 10.djvu/38

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28
THE POPULAR SCIENCE MONTHLY.

is guarded by sensitive tentacula and a sphincter muscle. The mouth is situated at the bottom of this branchial sac, down the side of which minute particles of food are swept by ciliary action, so as to be brought within the simple commencement of the œsophagus. The effete sea-water passes through the walls of this branchial cavity into a general body-chamber, in which the viscera are contained. This cavity is bounded externally by a muscular expansion, lining the outer cellulose tunic. By the periodical contraction of this muscular sac, the water which enters it, together with food-residues and ova, is expelled through another funnel-like opening, adjacent to and very similar to that by which it gains entry to the branchial chamber.

Although these ascidians have a definite alimentary canal, a circulatory system, and respiratory organs, together with a distinct genital apparatus, their life of relation with the external world is of the simplest description. They are stationary creatures, and have no prehensile organs, food being brought to the commencement of their alimentary canal by ciliary action.

In correspondence with such a simple mode of life, we might expect to find a very rudimentary nervous system, and this expectation is fully realized. The Tunicata possess a single small nervous ganglion lying between the bases of the two funnels through which water is taken in and discharged. This ganglion receives branches from the tentacula guarding the orifice of the oral funnel, and possibly from the branchial chamber, while it gives off outgoing, filaments to the various parts of the muscular sac, and perhaps to the alimentary canal, and some of the other internal organs. In some of the solitary Tunicata a rudimentary visual function is presumed to exist. At all events, pigment-spots are situated on, or in very close relation with, the solitary ganglion. This single body seems to serve for the performance, in a rudimentary manner, of the various functions discharged by at least two pairs of ganglia in a large number of higher Mollusca, viz., those known as the cerebral and the parieto-splanchnic or branchial.

The brachiopods are among the oldest and most wide-spread of the forms of life in the fossil state, and the geographical distribution of their living representatives at the present day is also very wide. Like the Tunicata, they are headless organisms, and lead a sedentary existence, attached either by a pedicle or by one division of their bivalve shells. The mouth is unprovided with any appendages for grasping food—nutritive particles being brought to it by means of ciliary currents. Numerous muscles exist which connect the valves of the shell to one another, and with the inclosed animal. And, though the visceral organization of the brachiopods is somewhat complex, no definite sense-organs have yet been detected in any of them. In the nervous system of these sedentary animals, there is, therefore, nothing answering to a brain as it is ordinarily constituted,