146 Elementary Zoology. 



outside, and this is followed by two muscular layers, an outer 

 layer of circularly arranged fibres and an inner layer of 

 longitudinal fibres. The epidermis is the epiblast of the 

 embryo ; the layers below are formed by the outer walls of 

 mesoblastic blocks, the innermost layer being the coelomic 

 epithelium. The body-wall of the earthworm is sometimes 

 spoken of as the dermo-muscular tube. In the crayfish the 

 simple structure of the body-wall is more complicated. In the 

 first place the empidermis, instead of secreting a simple trans- 

 parent cuticle, produces a much thicker cuticle, which is for the 

 most part calcified. The two muscular layers are no longer 

 recognizable in their simplicity. Instead of two continuous 

 sheets of muscle partly interrupted at the septa, which is found 

 in the earthworm, the muscles of the crayfish are broken up 

 into individual masses running in different directions, in which 

 the regular arrangement is no longer visible. Furthermore, 

 there is between the muscles and the epidermis a layer of 

 connective tissue, the dermis, not represented in the earthworm.' 



The immense thickness of the muscular layers has re- 

 sulted in the nearly complete obliteration of the coelom, as it 

 has also in the cockroach, which from the present point of 

 view is essentially like the crayfish. The same, too, is the case 

 with the anodon and with the snail. 



In the Vertebrata all the structures lying outside of the 

 ccelom may be regarded as collectively equivalent to the 

 dermo-muscular tube of the earthworm. 



In both cases it is the product of the somatic mesoblast. 

 But in the vertebrate the complication of this body-wall is 

 much greater. 'The epidermis is thicker, and is among ver- 

 tebrated animals modified in many -different ways, some of ' 

 which have been dealt with on p. 112, et seq. The same state- 

 ment applies to the dermis. The muscular layers are divided 

 up into separate muscles which have often characteristic 

 arrangements serving to separate the great groups of vertebrates 

 from each other. In the fishes, for example, there is a more 

 regular and apparently primitive arrangement. The muscles 

 are to some extent arranged in continuous sheets, only 

 ' A dermis lias been found in certain earthworms 



