INTEGUMENT. 805 



even be separated into several layers. It is of considerable thickness 

 in many Annelida (Clicetopoda), and may be perforated by pores. Cilia 

 are found principally in the larval stages of Platylielmintlies and 

 Annelida. Where there are no cilia, the superficial cuticular mem- 

 brane, which may project in the form of tubercles or spines, consists 

 of a substance allied to the chitin of the Arthropod skin, like which it 

 may bear cuticular formations of many kinds, such as hairs, bristles, 

 hooks, etc. In many Nemathelminthes, as well as in segmented 

 worms, this firm cuticula gives rise to a kind of exo-skeleton, which 

 opposes the contractions of the dermal muscular envelope. In the 

 Chcetopoda among the Annelida, but also in the Rotifcra, the tough 

 integument is divided into a number of sections lying one behind the 

 other. These, like the segments of Arthropoda, are connected by 

 soft portions of skin and moved by the dermal muscles, which are 

 divided into corresponding groups. The cutaneous segments of the 

 Rotifera are not true metameres, since there is no segmentation of 

 the internal organs. 



Cutaneous glands are very widely distributed ; they are sometimes 

 unicellular, sometimes poly cellular, and are sometimes situated 

 directly ^beneath the epidermis, sometimes in the deeper tissues of 

 the body. 



The tissue which lies beneath the hypodermis, and which we may 

 call the cutis, contains in all cases longitudinal and in some cases 

 also circular muscles, and so constitutes a muscular cutaneous envelope, 

 the principal locomotory organ of worms. Taking into consideration 

 the importance of this dermal muscular system in the locomotion of 

 worms, we must attribute a certain systematic value to the special 

 forms which it assumes, a value which, however, we must be careful 

 not to exaggerate. The stratification and the direction of the fibres 

 of this dermal muscular system is most complicated in the Platyhel- 

 minthes and in the Hirudinea amongst the Chcetopoda, for here we 

 find the circular and longitudinal muscles, which are embedded in a 

 basis of connective tissue, crossed by muscle fibres, which run in a 

 dorso-ventral direction (sometimes also by fibres running obliquely). 

 To these may be added groups of muscular fibres, which serve to 

 attach the internal organs to the integument. The suckers of the 

 parasitic worms, the pits and the parapodia with their setae of 

 (7Aeeopo<&,mustbe looked upon as special differentiations of the dermo- 

 muscular envelope. These aids to locomotion are mostly developed 

 upon the ventral surface. The suckers and their accessory hooks, 

 etc., are situated either near the two ends or in the middle of the 



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