INTEGUMENT OF VERMES. 



137 



partSj we find tlie muscular portion to be, as a rule, tlie larger ; and 

 the layer whicli corresponds to the true integument to be propor- 

 tionately feebly developed. 



The proper integument is formed, as a rule, of a layer of cells, the 

 elements of which are often so slightly separated that^ they form a 

 syncytium. This layer corresponds to an epidermis. In the 

 Turbellaria it is everywhere provided with cilia. In many the 

 cilia are placed on an apparently homogeneous layer, which resembles 

 a cuticle. But the cilia must be regarded as processes of the cells. 

 Even in those forms, such as the Cestoda, in which there are no 

 cilia in the adult, there is an investment of cilia during the embryonic 

 stages. The embryos of the Trematoda also have it. In many 

 Annelids there are ciliated spots at various parts of the body, or 

 large tracts may be clothed with cilia. 



The part that this investment of cilia plays in locomotion is best 

 seen in the smallest forms. In the young state it is generally the sole 

 organ of locomotion. By the growth of processes of the body the 

 cilia-bearing surface is increased, and so the cilia become of greater 

 importance in locomotion. This is their character in the larvfe of 

 the Gephyrea and of most Annelids. The cilia are arranged on 

 ridge-like processes, which surround certain tracts of the surface of 

 the body, as lines or circlets of cilia; the arrangement of these 

 is generally characteristic of the various divisions. One or more 

 circlets of cilia surround the body ; and by these the larvae of the 

 Chsetopoda are divided into mesotrochal, telotrochal, and poly- 

 trochal forms. Even if the surface of the body bears other cilia also, 

 those of the circlets are more powerfully developed, and their 

 action essentially aids in more rapid locomotion. Of these circlets of 

 ciha (Fig. 57,0 Dv) one is more remarkable than the rest, it appears 



Fig. 57. Arrangement of the ciliated bands in the larva? of the Echinoderma, 



A B, and of the Vermes, CD. v Anterior ; w Posterior circlet of cilia. 



Mouth, t Enteric canal, a Anus. 



in the very earliest stage, and divides the body into an anterior and 

 a posterior portion. The former represents the upper part of the 

 future head of the worm, while from the other portion the whole of 

 the rest of the body is developed, The primitive circlet of cilia 



