ANNELIDA 



87 



mesoderm, both primary and secondary, may thus be formed, 

 but the ectoderm may contribute to the former. Mesenchyme 

 cells are budded off which give rise to the products of the 

 primary body cavity, and the rest of the mesoderm forms the 



Ectoderm 

 Mesoderm 



Primary 

 mesoderm cell 



Mouth 

 Nerve cord 



Primary cells of : 

 (a) Mesoderm 



>(&) Nephridia 

 \c Nerve cord 



FIG. 42. Lumbricus. Developmental stages, after Wilson. The first three 

 figures illustrate the remarkably constant manner of segmentation in 

 Oligochaets, Mollusca, Rotifera, and other Trochozoa. The view in each 

 case is a dorsal one : B is anterior, D is posterior, A is left and C is 

 right. The remaining figures illustrate, 1, the flattening of the blastula 

 and the position of the special product of D relegated to form the meso- 

 derm ; 2, the gastrula, and by dotted lines the extension of the mesoderm 

 on each side of the enteron ; 3, the blastopore narrowed to be carried 

 in by the stomodeal invagination, the final opening to the exterior being 

 the mouth. The last figure also indicates the position of the mesodermal 

 band, the nephrogenous band, and the neuroblastic band, which are all 

 paired. 



coelom. It is at first solid, but as it grows and spreads spaces 

 appear which fuse to form the coelom, and the nephridia also 

 become hollowed out and establish openings into it, the pro- 

 cedure going on from the anterior to the posterior end. The 

 anterior ones are protonephridia with solenocytes which project 

 into the head schizocoel, not into the coelom, and they are 

 developed from special ectoderm cells in the region of the dorsal 



