142 COMPARATIVE ANATOMY. 



Skeleton. 

 § 112. 



When somewhat firmer than usual the integument in many divi- 

 sions of the Vermes plays an important part as an organ of support ; 

 these relations have been already referred to. Organs which possess 

 this function, without any subsidiary relations, are more worthy of 

 consideration. Supporting organs of this kind are seen in the 

 cartilaginous pieces in the cephalic segment of a number of tubi- 

 colous Annelides ; from these pieces processes pass out and ramify, 

 as fine bands, in the feather-like plates. This is to be regarded 

 as the formation of an internal skeleton, but it presents analogies 

 only to other similar arrangements. 



This also holds for the branchial skeleton of the Enteropneusti, 

 which is made up of a lattice-work of homogeneous rods (cuticular 

 structures). Its arrangement and development call to mind the 

 branchial skeleton of the lowest Vertebrata (Amphioxus), but it 

 cannot be said to have any very close relations to it. 



Muscular System. 



§ H3. 



The muscular system of the Vermes is connected with the integu- 

 ment, and forms in most of them the largest part of the covering of 

 the internal organs. In some it is only slightly developed. The 

 general arrangement of the fibres follows one of several types, which 

 may be thus characterised : 



1) Circular, longitudinal, and radial fibres form a connected mass 

 of muscle, in which the two former are separated into layers, and are 

 traversed by the radial fibres. The circular fibres form an outer and 

 an inner layer, between which the longitudinal fibres are placed. 

 The radial fibres run from the interior of the body to the surface. 

 At the lateral edges of the body they pass directly from the dorsal 

 to the ventral surface. This arrangement of the muscles is found in 

 the Platyhelminthes and Hirudinea. In addition to these muscles 

 there are fibres which run obliquely ; but they are not present in the 

 Nemathelminthes nor in the Turbellaria rhabdoccela. 



2) The longitudinal fibrous layer is alone present. This is the 

 case in the Nematodes, Chastognathi, and in Polygordius. The 

 longitudinal muscles are distributed in various ways. The muscular 

 fibres either pass directly below the epidermal layer (matrix of the 

 cuticle) in the form of flat bauds, the broad sides of which are 

 approximated, or they have their edges approximated, and their 



