. Principles of modern biology. Biology. Fig. 16-2. Section of o flagellated chamber of a sponge. mote from the source of absorption. The wall of the sponge is composed of only two cell layers, although some sponges have a large amount of relatively inert material, the mesoglea, interposed between the outer and inner cell layers. In commercially valuable sponges (Fig. 16-3), the mesoglea is composed of a silklike protein substance (spongen); but in others the skeletal material is calcare- ous or silicaceous, according to the species. THE SACCULAR DIGESTIVE SYSTEMS OF HYDRA AND PLANARIA In simp
. Principles of modern biology. Biology. Fig. 16-2. Section of o flagellated chamber of a sponge. mote from the source of absorption. The wall of the sponge is composed of only two cell layers, although some sponges have a large amount of relatively inert material, the mesoglea, interposed between the outer and inner cell layers. In commercially valuable sponges (Fig. 16-3), the mesoglea is composed of a silklike protein substance (spongen); but in others the skeletal material is calcare- ous or silicaceous, according to the species. THE SACCULAR DIGESTIVE SYSTEMS OF HYDRA AND PLANARIA In simplest form, the saccular type of di- gestive cavity is found in the fresh water polyp, Hydra. Essentially this small familiar aquarium animal has the form of a sac, in which one opening, the mouth, communi- cates with the surrounding water (Fig. 16-4). The body wall of Hydra is composed of only two cell layers; but between the outer ecto- dermal epithelium and inner endodermal epithelium, there is a thin layer of nonliving matrix, the mesoglea. Hydra captures a variety of small free- swimming animals by means of tentacles, a group of slender, mobile, threadlike organs that originate from the body in the region surrounding the mouth (Fig. 3-12). Each ten- a paralyzing fluid into such victims as may-. Fig. 16-3. Yellow sponge. The skeletal material of this sponge is spongin, a silklike compound. (The ascu- lum is at the top.) (Courtesy of the Fish and Wildlife Service, Department of the Interior.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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