iv VERMESDERMO-MUSGULAR TUBE 193 



the blood- vascular system may penetrate as far as into the hypodermis, 

 and so enter the service of the general cutaneous respiration. 



In Chcetopoda, a plexus of ganglionic cells lying immediately under 

 the hypodermis can be demonstrated, which is connected by nerve 

 fibrillae with the thread-like cells of the hypodermis. 



In many worms (Aphanoneura, Archiannelida, Saccocirrus, Opheliacea, 

 various Polychceta of families widely separated, and further in the 

 Priapulidce, Phoronidce, and Sagitta) the central nervous system lies in 

 the hypodermis in such a way that no sharp distinction can be seen 

 between the usual hypodermis cells and the nerve elements. In many 

 Annulata whose central nervous system lies under the hypodermis, 

 the former nevertheless passes into the hypodermis at the most anterior 

 and posterior ends, the anterior part of the brain into the hypodermis 

 of the prostomium, and the most posterior end of the ventral chord 

 into the hypodermis of the tail segment. 



The sensory organs of most worms, which will be described else- 

 where, belong to the hypodermis. 



The hypodermis is often separated from the underlying tissues by 

 a thin supporting or basal membrane. 



III. The Dermo-museular Tube. 



Immediately under the outer integument in most worms the body 

 musculature lies in the form of a dermo-muscular tube, which repeats 

 .the shape of the body. It consists in general of two well-developed 

 layers, an outer layer of circular fibres and an inner layer of longi- 

 tudinal fibres. 



These two layers are found in all Nemertina, except Cephalothrix, 

 where the circular muscle layer may be wanting. In the Schizonemertina, 

 and further in Polia and Falencinia, there is, in addition, an outer longi- 

 tudinal layer, which may even be more strongly developed than the 

 inner layer; in Carinina, Carinella, and Carinoma there is also an addi- 

 tional circular layer. All these layers form in the Nemertina a con- 

 tinuous tube, nowhere broken through in any way worth mentioning. 



Among the Nemathel /ninths, all the Nematoda possess only the 

 longitudinal muscular layer (Fig. 127, Im). This is broken through 

 in 4 lines running in the longitudinal direction, and thus falls into 

 4 longitudinal portions. Two of the 4 longitudinal lines are median 

 (dorsal and ventral) and 2 lateral. In these lines of interruption the 

 subcuticular granulated dermal layer (hypodermis) is thickened, and 

 in it lie definite organs of which we shall speak later. In the Gordiidce 

 the ventral line only is clearly marked. 



In the Acanthocephala, besides the longitudinal muscular layer, 

 an outer circular muscular layer is added. 



In the Annulata the dermal musculature almost everywhere 

 appears in the typical form. The circular layer is very rarely 

 VOL. i o 



