IKTEGUMENT OF VERMES. 141 



from the body- wall is attached to tlie bliud cud of the tube; 

 it is to be regarded as a retractor. lu many Nemertiua (Liueus, 

 NemerteSj etc.) the stylets are absent. 



In some the tube is small (Polia involuta), and so far resembles 

 the structures iu other PlatyhelmintheSj which may perhaps be 

 regarded as the first stage towards the highly differentiated pro- 

 boscis of the Nemertiua. Such are the stylets present at the 

 anterior end of the body of the Cercarite, which serve for borings and 

 are placed either on the surface, or at the bottom of a deep folli- 

 cular depression. Lateral stylets having the same relation to the 

 median one as in the proboscis of the Nemertiua are often observed ; 

 so that we may conclude that primitively there was the same 

 organisation in this respect in a large division of the Platyhelminthes. 

 Even in certain Nemathelminthes we find similar ai'rangements, so 

 that we have here to do with a wide distribution of similar characters. 

 In some this arrangement is only found in the young stages, and 

 disappears in the adult organism (Trematoda) ; in others it not only 

 persists, but is connected with important differentiations (Nemertiua) . 



§ 111. 



The integument of the Vermes attains to a higher position 

 through the differentiation of glands as special organs of secretion. 

 Organs of this kind have been recognised in nearly all the divisions 

 of the Vermes, and are very common among the Annulata. In most 

 cases they appear to be unicellular, and sometimes lie immediately 

 beneath the integument, and sometimes, when a distinct coelom is 

 wanting, in the deeper parts of the body. 



Among the Platyhelminthes, unicellular dermal glands are known 

 in the Trematoda. They are generally placed in groups in the 

 anterior part of the body, and in the hinder part also, where they are 

 connected with the suckers. The glands are greatly developed in 

 the Hirudinea, and especially in the Blood-leeches, where they are 

 scattered in the parenchyma of the body, and open on to the skin by 

 long ducts. They appear to be developed in relation to the gene- 

 rative function. Unicellular glands are also present in the integu- 

 ment of the Scoleina, where they are placed between the cells of the 

 matrix. In many cases the glands take up a deeper position, and 

 their ducts, only, pass between the epidermic cells. 



In the Gephyrea tubular glands also are connected with the 

 integument, and tubes are also found in the Annelides (Fig. 58, d). A 

 glandular layer is developed on one portion of the body of the 

 Lumbricidte, as a clitellus; but this organ does not appear to be so 

 simple in structure, for the tubes are invested by a special epithelium, 

 and are sometimes lobate in form. Glandular tubes, containing 

 masses of rod-shaped bodies, are very common among the Chtetopoda 

 (Spio, Aricia), In the Nemertiua there are also glands which secrete 

 a viscous fluid. In many cases the secretion of the dermal glands is 

 used to form an investment for the ova. 



