Mantle 120 STRUCTURE OF CONCHIFERA OR BIVALVES. terminating near one of the canals or siphons, which also carries out the water that has been taken in through the other for the purposes of respiration. The figure also shows the large fleshy foot, by which this animal can move itself along Respiratory Tubes. Fig. 62.-ANATOMY OF MACTRA. the ground, or bore into sand or mud. The heart and circu lating system are less complete than in the Gasteropoda; but are far higher in character (as are most of the other parts Anus Muscles STRUCTURE OF TUNICATA. 121 of the nutritive apparatus) than the corresponding parts in Articulated animals, in which the apparatus for locomotion so much predominates. 114. The group of Acephalous Mollusks which are destitute of the power of forming a shell, includes two classes, of which one does not depart widely from the general Molluscan type, whilst the other presents so strong a general resemblance to Zoophytes, that until recently it has been universally ranked with it. The first of these classes receives its name TUNICATA from the circumstance that the mantle, instead of secreting a shell, is very commonly condensed into a tough leathery or cartilaginous tunic. Many of these animals live separately, and have the power of freely moving through the water. Others are associated in compound masses, of which, however, the individuals are not connected by any internal union. But others form really composite structures, like those of Zoophytes (§ 124); each individual being able to live by itself alone, but being connected by a stem and vessels with the rest. The general structure of the individuals is the same, however, in the single and in the composite animals of this class, and may be understood from the accom panying figure (fig. 63). The cavity of the mantle possesses, as in the former instance, two orifices; by one of which, b, a current of water is continually entering, whilst by the other, a, it is as continually flowing out. These orifices lead into a large chamber, the lining of which, folded in various ways, constitutes the gills; and at the bottom of this chamber lie the stomach, e, and the intestinal canal, i, which terminates near the aperture for the exit of the water. All these parts 122 STRUCTURE OF TUNICATA AND POLYZOA. are covered with cilia, by the action of which a continual stream is made to flow over the gills and to enter the stomach; and the minute particles which the water brings with it, and which are adapted to serve as food, are retained and digested in the stomach. Even these animals, fixed to one spot during all but the early part of their lives, and presenting but very slight indications of sensibility, possess a regular heart and system of vessels; and these vessels form part of the stem, t, by which the compound species are connected. A single nervous ganglion is found between the two orifices; this seems to receive sensory fibres from tentacula situated around the oral orifice, and to transmit motor filaments to the muscular coat which underlies the outer tunic, so that any irritation applied to the former occasions a contraction of the latter, which tends to expel the offending particle.-This class is one of particular interest to the naturalist, since we see in it the tendency to the formation of compound structures, by a process resembling that of the budding of plants, which is essentially characteristic of Zoophytes; this tendency, however, is more fully manifested in the succeeding class. 115. The animals forming the class POLYZOA (more commonly known as Bryozoa) are seldom or never found solitary; since, in consequence of their universal tendency to multiply by gemmation, they form clusters or colonies of various kinds. The body of each individual is inclosed in a sheath or "cell," which is sometimes horny, sometimes calcareous; and the composite skeleton formed by the aggregation of these, which has sometimes a branching or leaf-like form, but sometimes possess the compactness of a stony coral, is known as the "polyzoary." In their general structure the animals of this class possess considerable analogy to the Tunicata; but the Molluscan type presents itself under a more degraded aspect, no vestige of a heart or of blood-vessels being here discernible, and the general structure being so simplified as to manifest no great degree of elevation above that of Polypes. The typical structure of these animals may be understood from that of the Bowerbankia (fig. 64), which is one of those whose cells are not in contact with each other, but grow forth at intervals from a creeping stem. The mouth, a, is situated in the midst of a circle of arms fringed with cilia; these STRUCTURE OF POLYZOA AND RADIATA. 123 arms do not serve, however, like those of polypes, to grasp the food; but the vibration of their cilia produces a powerful current which brings both food and oxygen. The mouth leads by a large funnel-shaped œsophagus or gullet, to a gizzard, b; in which the particles of food that enter it are ground down, by the action of its muscular walls and of the tooth-like processes that line it. Below this gizzard is the true digestive stomach, c, around which the rudiment of a liver may be traced; and from this stomach there passes upwards an intestinal tube, which terminates by a distinct orifice at d, on the outside of the circle of arms. The digestive apparatus is evidently formed, therefore, upon a much higher plan in these animals than it is in the true polypes, which have no true anal orifice. The Molluscan character of these animals is further shown by the presence of a single nervous ganglion, situated between the two orifices, as in the Tunicata; this acts upon a complex a, œsophagus; b, giz- apparatus of muscles, by which the animal d, orifice of intes- can be either drawn into its cell or projected forth from it, with great rapidity. Fig. 64. BOWERBANKIA. zard; c, stomach; tine. 116. The fourth subdivision, that of RADIATA, includes those animals which have the parts of the body arranged in a circular manner around a common centre, so as to present a radiated or rayed aspect. This arrangement is well seen in the common Star-fish (fig. 65), which has five such rays, all having a precisely similar structure, and thus repeating each other in every respect. The mouth of this animal is in the centre; and it opens into a stomach, which occupies the central disk, and sends prolongations into the rays. The nervous system is, in like manner, composed of a repetition of similar parts. A plan of it is seen in fig. 66; where a shows the position of the mouth, which is surrounded by a ring or nervous cord, having five ganglia, corresponding to the five arms. From each of these ganglia proceeds a branch along its arm, terminating in a little organ at its extremity, which is believed to be an imperfectly-developed eye. No other organs of special sense can be detected in any of these ani 124 STRUCTURE OF RADIATA. mals; and it is only in a few that even these imperfect eyes can be discovered. In the inferior Radiata, not the slightest Fig. 65.-SHELL OF STAR FISH. traces of a nervous it is very doubtful system have yet been discovered; and whether any such structure exists in them. It is only among the higher Radiata that any locomotive power exists; and this is usually so feeble that the animals remain in the same locality during the greater portion of their lives. Generally speaking, there is a period in the history of each species, in which there is a more active movement, that serves to prevent the accumulation of individuals in one spot; but this movement is of a purely automatic character, rather resembling that of the "zoospores" of plants, than the intentional change of place of the higher animals. Fig. 66.-NERVOUS SYSTEM |