40 COMPARATIVE ANATOMY. 



§ 33. 



The earliest musculature of the body is closely related to the 

 integument, from which it can with difficulty be separated. Since 

 this is the case not in the Coelenterata alone, we have here an 

 instance in favour of an essentially equivalent origin for this part of 

 the musculature in all cases. Together with the integument, it 

 forms, on the appearance of a body- cavity, a " dermo-muscular 

 tube/ 5 which encloses the other organs. The arrangement of 

 the muscular fibre seldom presents much regularity till the body 

 becomes jointed into separate parts, placed one behind the other 

 (metameres) ; and with the development of organs of support the 

 muscles become differentiated into separate groups. Collections 

 of fibres form bundles, and these again make up larger complexes, 

 muscles. The segmentation of the muscular system corresponds 

 therefore to the segmentation of the body, and the separate 

 segments differ in proportion to the difference in function of the 

 metameres. The various kinds of movement which are produced 

 by the crossing of the fibres of the dermo-muscular layer in 

 different strata, are, where the muscular system is more highly 

 differentiated, effected by groups of muscles acting in opposition 

 to one another, and completely balancing one another in their 

 action. 



Locomotion by movement of the whole body is brought about by 

 the dermo-muscular tube and the differentiations which arise from 

 it ; the whole integument, in the first instance, takes a share in this 

 activity. A further differentiation arises from this state of things 

 when special appendages are formed, as limbs, on certain parts of 

 the body. When the animal changes its place these act as the arms 

 of a lever. They have the form of simple soft processes of the 

 dermo-muscular tube (Ringed worms), or of jointed organs, which 

 are supported by the integument (Arthropoda), or by means of internal 

 skeletal structures ( Vertebrata) . The complication of the muscular 

 system is in close connection with the development of supporting 

 organs ; and the two form a single locomotor system in which the 

 skeleton plays the passive part. 



d) Nervous System. 

 § 34. 



In the lowest conditions of animal organisation the protoplasm of 

 the cells is the seat of sensation, as well as of movement ; and this 

 is permanently the case in the lowest organisms. As the muscular 

 layer of the body becomes differentiated, the ectoderm becomes the 

 principal organ of sensation. The differentiation of a nervous 

 system is due to the further development of a portion of this layer 



