. Elements of Comparative Anatomy. 58 COMPAEATIVE ANATOMY. — c Fundamental Forms of the Animal Body. § 49. Owing to tlie infinite variety of tlie external cliaracters of animal organisms, it is necessary to seek for fundamental forms to wliicli this variety may be referred. We must also ascertain the con- ditions which influence and give rise to the most important modifi- cations of these forms. These results may be obtained in diiSerent ways. We will choose the shortest by commencing with the lowest stage of the animal organism. This is the stage which the Gastrula form presents to us ; by it
. Elements of Comparative Anatomy. 58 COMPAEATIVE ANATOMY. — c Fundamental Forms of the Animal Body. § 49. Owing to tlie infinite variety of tlie external cliaracters of animal organisms, it is necessary to seek for fundamental forms to wliicli this variety may be referred. We must also ascertain the con- ditions which influence and give rise to the most important modifi- cations of these forms. These results may be obtained in diiSerent ways. We will choose the shortest by commencing with the lowest stage of the animal organism. This is the stage which the Gastrula form presents to us ; by its wide distribution this form will provide us with the characters which are best adapted for our purpose. An organism at this stage is some- what spherical or oval in shape, and the mouth will be found at a point on the surface. If we imagine an axis (Fig. 16, AB) drawn straight through the digestive cavity, the pole corresponding to the opening of the mouth represents the oral, and the opposite the aboral pole. This axis [AB) is the primary axis of the body. In a body of a regular cylin- drical or spheroidal shape we can imagine as many lines as we please drawn through the body perpendicular to this axis. (Secondary axes, ah, c cl.) In this instance they are all equivalent. The secondary axes are in this case indifferent to one another, and are cha- racteristic of a lower condition. The organism, either when moving freely in the water, or when fixed (by the aboral pole, of course), as it afterwards is, is differentiated by the develop- ment of a certain number of secondary axes, their development having relation to the main- tenance of the balance of the body. We here, then, have to do with a statical cause. The development of the organism along its secon- dary axes takes place through the development of external appendages, tentacles and the like, or through differentiation of the enteric cavity, or through the laying down of other organs (such as the generative glands) ia
Size: 880px × 2839px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
