ANNUL AT A 



233 



is brought into contact with the ova of this second worm, as 

 they pass from the ovary, where they are produced, to the egg-sac 

 or to the exterior. It is believed that in some instances at least 

 the genital ducts are modified nephridia. 



It will be a profitable practical exercise for the student to 

 make for himself a diagram of the sex organs of the earthworm 

 or some other hermaphrodite form by reference to several stand- 

 ard texts. 



273. Reproduction and Development. Sexual reproduction 

 is universal. As we have seen, copulation may occur or the 

 elements may come together in the water. In the Oligochaeta 



x mi. 3 



FIG. 105. Two stages in the development of Nereis. A, 8-celled stage; B~, lo-celled stage, 

 both viewed from the active or ectodermal pole, mi. 1 , mi,*, and mi.*, the first, second and third 

 sets of micromeres separated from ma., the macromeres; s 1 , first somatoblast, one of the second 

 group of four cells to be budded from the macromeres; s 2 , second somatoblast, one of the third group, 

 which gives rise to the mesoderm. The micromeres are ectodermal and the macromeres produce 

 the entoderm. (After Westinghausen.) 



and leeches the fertilized ova, or the ova together with masses 

 of spermatozoa, are enclosed in a cocoon of secreted material 

 and within this case the young worm is developed. In the 

 Polychaeta the larva undergoes its development in a free state. 

 Segmentation in Annulata is complete and usually unequal, 

 giving rise at the eight- celled stage to four micromeres and four 

 macromeres (Fig. 105). The micromeres produce the ectoderm; 

 directly or indirectly the macromeres give rise to the entoderm. 

 Early in the cleavage "primitive mesoblasts" cells which pro- 

 duce the mesodermal structures are separated from the macro- 

 meres. A gastrula is formed either by invagination or by over- 



