. Animal physiology. Physiology, Comparative; Physiology, Comparative. 106 STRUCTURE OF THE -Social Ascldi^e. compound structures, like those of the Polypes (§. 131); each i a 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 ani- mals of this class, and may be understood from the accompanying figure. The cavity of the mantle possesses, as in the former instance, two orifices; by one of which, 6, a current of water is continually ent
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 106 STRUCTURE OF THE -Social Ascldi^e. compound structures, like those of the Polypes (§. 131); each i a 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 ani- mals of this class, and may be understood from the accompanying figure. The cavity of the mantle possesses, as in the former instance, two orifices; by one of which, 6, 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 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 re- tained and digested in the stomach. Even these animals, fixed to one spot during all but the early part of their lives, and pre- senting but very slight indications of sensibility, possess a regular heart and system of vessels; and these vessels form part of the stem, £, by which the compound species are connected. 127. This class is one of particular interest to the naturalist; since many of the animals it contains bear so strong a resem- blance to certain of the higher Polypes, that it is difficult to separate them ;—thus showing a connexion between two groups apparently quite distinct, and exemplifying the principle that the different classes are not separated by wide gaps. This con- nexion is further manifested, in the extension of these com- pound structures, by a process resembling that of the budding of
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