. Animal studies. 50 ANIMAL STUDIES essentially the same structure but with various modes of branching (for some of the commoner modes, see Fig. 20). In some of the higher forms a division of labor has arisen among various members of the association which has led to most interesting results. For example, Fig. 22 represents a species of hydroid found investing the shells of sea-snails occupied by hermit crabs (Fig. 66). To the unaided eye its appearance is that of a delicate vegetable growth, but when placed under the microscope it is found to consist of a multitude of Hydra-like animals united
. Animal studies. 50 ANIMAL STUDIES essentially the same structure but with various modes of branching (for some of the commoner modes, see Fig. 20). In some of the higher forms a division of labor has arisen among various members of the association which has led to most interesting results. For example, Fig. 22 represents a species of hydroid found investing the shells of sea-snails occupied by hermit crabs (Fig. 66). To the unaided eye its appearance is that of a delicate vegetable growth, but when placed under the microscope it is found to consist of a multitude of Hydra-like animals united by a hollow branching root system con- necting the gastric cavities of all of them (Fig. 22). Certain individuals (a) with tentacles and a mouth resemble a Hydra; others, without a mouth and ten- tacles, are reduced to a club-like form (b) liberally supplied with nettle-cells upon their free extremi- ties; while the third type (c), likewise devoid of a mouth, possesses rudiments of ten- tacles below which are borne numerous clumps of repro- ductive cells. The first type, the only one possessing a mouth, captures the food, and after digesting it distributes the greater portion to the remaining members by means of the connecting root system; those of the second form, de- fending the others by means of their nettle-cells against the inroads of a foreign enemy, are the soldiers of the colo- ny; while the third type produces the eggs from which new individuals develop. In some of the higher Hydrozoa, the Portuguese man- of-war (Fig. 23), this division of labor has reached a more advanced stage of development, and in addition the entire. Fio. 22.—An enlarged portion of a hydroid colony {Ilydraclinia), showing (a) the nutritive polyp, {b) the defensive polyp, and (c) the reproductive 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 r
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Keywords: ., bookauthorjordanda, bookcentury1900, bookdecade1900, bookyear1903
