NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 9 



The presence of a mesoblastic muscle system and of a mesenchyme, 

 or parenchymatous layer between the ectoderm and the endoderm, 

 gives the annelid larva, or the platyhelminth adult, the status of a 

 triploblastic animal; but the middle layer is here only an elaboration 

 of elements present also in the so-called diploblastic coelenterates. 

 The young annelid larva, however, is endowed from its parents with 

 hereditary influences that will mold its growing tissues into structures 

 never attained by the coelenterates or flat worms. Particularly affected 

 are two individualized groups of mesoblast cells, which, though they 

 may be set apart in the platyhelminths, will give rise in the annelids 

 to special bands of mesoblastic tissue, known as the mesoderm. 

 Within the mesoderm will be formed a new body cavity, the coeloni, 

 and from the walls of the latter will be produced a new muscular 

 system, a more efficient excretory system, a circulatory system, and 

 various tissues of special functions, to all of which is added an exten- 

 sion and elaboration of the nervous system. With the formation of 

 the mesodermal cavities the triploblastic annelid larva becomes a 

 coelomate animal, but, shortly before the appearance of the coelom, 

 there takes place a segmentation of the body afifecting the ectoderm 

 and the mesoderm, so that the young annelid worm is almost at once 

 a segmented and a coelomate animal. 



II. THE MESODERM AND THE BEGINNING OF METAMERISM 



In the ontogeny of the articulate animals, the formation of the 

 coelomic cavities in the mesoderm is so closely associated with the 

 appearance of body segmentation as to give the impression that the 

 two are intimately related developmental processes, and since the 

 segmentation of the mesoderm is usually more conspicuous than the 

 segmentation of the body, embryologists often describe metamerism 

 in terms of mesoderm segmentation, as if the formation of "meso- 

 derm somites" were equivalent to body segmentation. Closer atten- 

 tion recently given to the sequence of events in the development of 

 the Polychaeta, however, shows that metamerism begins in the ecto- 

 derm and the primary ectodermal musculature, and that it secondarily 

 effects a division of the coelomic mesoblast into segmental sections. 

 Subsequently, the coelomic cavities are formed in the segmented 

 mesoderm. That coelomic sacs do not determine metamerism is 

 shown also by the formation of paired coelomic cavities in the preoral 

 cephalic mesoderm of the Onychophora and Arthropoda, in which 

 there is no corresponding external segmentation. 



Metamerism, therefore, probably took its origin in a subdivision 

 of the primary somatic musculature into successive sections (myo- 



