. Elements of Comparative Anatomy. 138 COMPARATIVE ANATOMY, persists in one division of the VermeSj tlie Rotatoria. While the posterior portion is differentiated into a more or less jointed body, the anterior part, which carries long cilia on a discoid thickening, is developed into a special organ, which is characteristic of this division. This wheel-organ—so-called from the movement of its cilia—varies greatly in character. It may be either permanently simple, and retain its primitive state, or it may be broadened out into lobate processes (Tubicolaria), or form tentacular prolongations (Step
. Elements of Comparative Anatomy. 138 COMPARATIVE ANATOMY, persists in one division of the VermeSj tlie Rotatoria. While the posterior portion is differentiated into a more or less jointed body, the anterior part, which carries long cilia on a discoid thickening, is developed into a special organ, which is characteristic of this division. This wheel-organ—so-called from the movement of its cilia—varies greatly in character. It may be either permanently simple, and retain its primitive state, or it may be broadened out into lobate processes (Tubicolaria), or form tentacular prolongations (Stephano- ceros), which frequently have a locomotor function in the larval stages only, and in the later fixed mode of life are used to bring food to the animal by means of the currents pi'oduced by the action of the cilia. In the Bryozoa, also, a circlet of cilia precedes the development of the tentacles, which are budded out internally to it. Owing to the position of the mouth, this circlet of tentacles does not resemble the more common form; but it nevertheless has close relations with the arrangements in some divisions, the Gephyrea, the larvae of which have a circlet of cilia surrounding the oral region. Further, in Polygordius, which except in this point resembles the Nematodes, there is a circlet of cilia; so that we recognise in this circlet of cilia an arrangement which may have been transmitted from an ancestral form common to many divisions of the Vermes. § 108. When cilia are absent the epidermic layer is covered by a cuticle, which varies greatly in character, and is a product of the secretion of the epidermic cells. This cuticle is a thin or even a soft layer in the Trematoda and Cestoda among the Pla- tyhelminthes. It has the same character in the An- nelida, but in them it may be very greatly developed (Fig. 58, c). It is also pre- sent in the Acanthocephali. When this layer is thickened pore-canals may be seen in it. In the class of the Nemat- helminthes
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