THE EARTHWORM 43 



somites are being formed behind the mouth, the cells of the 

 conjoined anterior ends of the bands become branched and 

 amoeboid, and ultimately arrange themselves so as to enclose 

 a single anterior median cavity, the prostomial or head cavity, 

 which is traversed by branched contractile cells. This anterior 

 cavity defines the prostomium, whose nature is therefore 

 different to that of the paired coelomic cavities which follow 

 behind the mouth. Later in development the partition be- 

 tween the prostomial and first pair of coelomic cavities breaks 

 down, as also do the septa between the first, second, and third 

 pairs of coelomic cavities, leaving the large anterior cavity in 

 which the buccal cavity and pharynx lie in the adult. 



Fig. 9, H, shows an embryonic worm in a tolerably advanced 

 stage of development. As the mesoblastic somites are formed 

 from before backwards, the anterior part of the worm is fully 

 segmented, the somites of either side having coalesced both 

 ventrally and dorsally. A little further back the somites 

 have coalesced ventrally but do not reach up to the dorsal 

 surface, and still further back they are wholly confined to the 

 ventral surface. Consequently, in the middle and posterior 

 regions of the embryo, the upper surface and sides of the 

 body consist only of the enteron covered over by a thin layer 

 of flattened epiblast. In the further course of development 

 the successive somites meet and fuse dorsally, and new somites 

 are continually formed by budding from the mesomeres at the 

 hinder end of the body. As the segmentation is completed, 

 the embryonic cells are differentiated to form the tissues of the 

 adult, and with the formation of the typical number of segments 

 the worm is completed. The anus is formed late in embryonic 

 life by an ectodermic invagination meeting and fusing with 

 the enteron in the posterior segment. 



There is every reason to believe that the earthworm is 

 descended from aquatic, and those in turn from marine 

 ancestors, and, therefore, the course of development has been 

 much modified in connection with the change from a marine 

 to a terrestrial habit. None the less, there are many in- 

 teresting lessons to be learned by the study of its development. 

 It is most important to remember that the segmentation of 

 the body, which is so characteristic a feature of the earthworm 

 and all Annelid worms, is at first internal and affects only the 

 mesoblastic bands. Only as the mesoblastic somites increase 



