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, e.g. 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 it is very greatly 

 developed, and is thicker than the subjacent matrix. Very often 

 several layers, differing from one another, can be made out in it; 

 they are formed of a substance which appears to be closely allied to 

 chitin. When separate portions of the cuticular investment are very 

 firm, a kind of dermal skeleton is formed in the Annulata, which is 



Fig. 58. Vertical section tliroiigh the integu- 

 ment of an Annelid {Sphferodorum). c Thick 

 cuticular layer with wide pore-canals, m Mus- 

 cular layer, m ' Muscles for the tuft of setse, 

 s, which occupies the ventral parapodium, 

 2), while the dorsal one, d, is represented by a 

 ^ ^ swelling, which contains glandular tubes. 



