144 COMPARATIVE ANATOMY CHAP. 



structure and a greater consistency, like the gelatinous tissue in 

 the Cnidaria. 



VI. The Musculature. 



The collective muscle elements of the Platodes may be brought 

 into 2 chief groups: (1) the general body musculature, and (2) the 

 special musculature of the organs, e.g. of the intestinal canal of the 

 copulatory organs, etc. The latter cannot here be taken into considera- 

 tion, as the musculature is adapted in every one of the extraordinarily 

 numerous cases to the special activities of the organ. 



The body musculature also is by no means so uniform throughout 

 the race as to make a generally applicable scheme possible. It 

 again falls into (1) the dermal musculature and (2) the dorso-ventral 

 musculature. The first lies under the basal membrane of the integu- 

 ment or under the outer cuticle, the second runs transversely through 

 the parenchyma between the various organs and connects opposite 

 points of the basal membrane or the cuticle. 



1. The dermal musculature is composed of layers, which are 

 generally clearly separated. In each of these layers all the 

 fibres run in a certain direction. We can distinguish longitudinal, 

 tranverse, and diagonal fibre layers. The diagonal fibre layer 

 is naturally always double. The longitudinal and the transverse 

 layers may also be double. We find the largest number of layers 

 5 or 6 in the Polydada ; in the Triclada their number is 

 smaller, we here find outer circular and inner longitudinal fibres, 

 between which diagonal fibres may be intercalated. The same is the 

 case in the BJiabdoccelidce, in which the musculature is weaker than in any 

 other Turbettaria. It is generally much weaker on the dorsal than on 

 the ventral side, on which the animals creep. Certain dorsal muscle 

 layers may be altogether wanting. In the Turbellaria the diagonal 

 muscle layer seems always to lie between the others. 



The sequence of layers in the dermal musculature of the Trematoda 

 is as follows : the circular or tranverse muscle layer lies externally ; 

 then follows a strong longitudinal muscle layer ; and inside comes the 

 diagonal muscle layer. In the Cestoda the diagonal muscle layer is 

 replaced by a strong inner circular muscle layer, lying pretty deep 

 under the skin, and divided from the outer circular layer and the 

 longitudinal muscle layer by a layer of parenchyma. 



2. The Dorso-ventral or Sagittal Musculature. Its fibres are 

 branched at both ends (Fig. 47, d, p. 47), and run through the 

 parenchyma from the dorsal to the ventral surface. Where intestinal 

 diverticula are developed, the fibres naturally run between them 

 as muscle septa, filling up the spaces. Where, as in the broad disc- 

 shaped Polyclada, the ramifying intestinal branches of the stomach 

 intestine radiate out towards the circumference on all sides, the septa 

 project more or less far from the latter towards the former, and where 



