SECT, x . PHYLUM ANNULATA 443 



like space, the beginning of the lumen of the middle part of the 

 alimentary canal (o3sophagus and intestine, int), the cells subse- 

 quently giving rise to the enteric epithelium. This canal becomes 

 continuous in front with the stomodaeum, and behind with a 

 second smaller ectodermal invagination, the proctodceum, which 

 arises in the position of the former pigment-area. The part of 

 the larva behind the prototroch now elongates, and two pairs of 

 invaginations, the setigerous sacs (set. sacs), appear at its sides : in 

 the interior of these, to which a third pair is soon added, are 

 developed setae which grow out to a great relative length as the 

 larval or provisional setce. Constrictions soon appear marking off 

 the first three segments, and at the same time the mesoderm bands 

 undergo a corresponding division into three pairs of mesoderm 

 segments. The mesoderm segments of each pair grow inwards 

 towards one another and surround the alimentary canal : in the 

 interior of each appears a cavity which is the beginning of a 

 segment or chamber of the ccelome. As the two mesoderm 

 segments become closely applied to one another and unite around 

 the alimentary canal, their two cavities also come into close 

 relation, and eventually are separated from one another only by 

 thin vertical septa, forming dorsal and ventral mesenteries which 

 subsequently disappear. Successive mesoderm segments also 

 come into close relationship with one another, their cavities 

 eventually only remaining separated by thin transverse partitions, 

 which form the intersegmental septa. 



The region in front of the prototroch becomes modified to form the 

 prostomium of the adult. The part immediately behind forms 

 the peristomium, which bears setae, and is to be looked upon as 

 the specially modified first segment. The body increases in length, 

 and additional segments with their setigerous sacs become dis- 

 tinguishable (E), until, on the development of the tentacles, the 

 outgrowth of the parapodia (para) with their cirri and the 

 permanent setae (which replace those first formed), the formation 

 of the full number of segments, and the completion of the internal 

 organs, the adult condition of the worm is attained. 



6. THE EARTHWORM (Lumbricus). 



General External Features. The Earthworm (Fig. 363) 

 has a long narrow body, which may be described as approximately 

 cylindrical, but slightly depressed towards the posterior end. 

 Dorsal and ventral surfaces are readily recognisable, the latter 

 being much paler in colour than the former, and exhibiting a 

 slight flattening ; the anterior end is distinguishable in the living 

 animal as that which is directed forwards in the ordinary creeping 

 movements of the worm. The surface, as in the case of Nereis, 

 is very distinctly marked out into segments or metameres by a 



