76 INVERTEBRATE ZOOLOGY 



In most instances, the ducts which carry the germ cells from 

 the ovaries and spermaries are at least in part (Fig. 47) derived 

 as modifications of excretory ducts. In the marine annelids 

 the varied relations of ducts for elimination of waste and libera- 

 tion of germ cells are well illustrated. In some forms the gono- 

 ducts and the excretory ducts are wholly independent (Fig. 47, 

 III), while in other forms the gonoducts or ciliated groove may 

 become united with either a protonephridial tube (II) or a 

 metanephridium (IV). 



Vasa deferentia of the male are usually directly continuous with 

 the gonads, while the oviducts of the female have communication 

 with the ovaries only through the body cavity. A portion of the 

 oviduct of the female is frequently enlarged as a uterus within 

 which the eggs are held for either a brief time or through the full 

 period of the development of the embryos. 



Following copulation, sperm cells are frequently stored within 

 the body of the female in a receptaculum seminis from which they 

 escape to fertilize the eggs which are produced over a considerable 

 period of time. The vitellaria are also accessory glands of the 

 female. They furnish nutritive materials and in some instances 

 supply the substances of which the eggshell is formed. In many 

 animals, there are no separate vitellaria and frequently in these 

 only a portion of the ovarian cells become functional ova, while 

 others act as nurse cells or follicle cells which supply the ova 

 with additional reserve food material. 



Copulatory organs are of common occurrence. An intromit- 

 tent organ known as the cirrus is frequently characteristic of the 

 male. This may be a modification of the terminal portion of the 

 vas deferens or, as is often the case, may be purely accessory 

 structures such as spicules or appendages modified to aid in the 

 transfer of sperm. Clasping organs are especially characteristic 

 of the males of some arthropods, and in the same group oviposi- 

 tors are frequent accessories of the female system. 



Locomotor System. — Numerous specialized structures for 

 locomotion are encountered in the single-celled organisms. 

 Cilia, flagella, pseudopodia, and myonemes are characteristic 

 partial differentiations of cells for movement. The first two of 

 these especially are carried over into the Metazoa as modifications 

 of epithelial cells for locomotion or the production of movement. 

 Many larval Metazoa perform locomotion by means of cilia exclu- 

 sively, but the flatworms and the rotifers represent the highest 



