130 INVERTEBRATE MORPHOLOGY. 



globular bodies whose ducts unite to form two vasa deferentia 

 opening to the exterior through a muscular intromittent organ, 

 and sometimes dilating to form reservoirs, the seminal vesicles, 

 in which spermatozoa may be stored up until required for 

 fertilization. The female apparatus is somewhat more com- 

 plicated. The ovaries are usually two in number and their 

 products pass to the exterior through special tubes, the ovi- 

 ducts, which may be exceedingly long and with the terminal 

 portion dilated to form a uterus in which the ova may pass 

 through certain stages of their development. Connected with 

 the oviducts there is usually a pouch-like structure, the semi- 

 nal receptacle, for the reception of spermatozoa, and further- 

 more they may receive the products of two other glands 

 which supply the yolk and the shell for the ova. The yolk- 

 glands are in some cases very voluminous, forming what is 

 termed the vitellarium, and have been apparently developed 

 by the separation of a portion of the original ovary, their 

 cells, which manufacture the yolk material, being accordingly 

 equivalent to germ-cells. The evidence for this supposition 

 is derived from the arrangement found in some Turbellaria 

 and will be pointed out, together with the variations which 

 the complex of organs presents, in the descriptions of the 

 various groups. 



I. CLASS TURBELLARIA. 



The Turbellaria derive their name from the fact that the 

 ectoderm is furnished with cilia, which form the locomotor 

 organs of the animals, whose gliding motion over tbe sur- 

 face of the objects among which they live is very charac- 

 teristic. The majority of the members of the class lead a 

 free life, some in fresh and some in salt water, and some even 

 on laud, creeping about on the under surfaces of stones or 

 weeds. A few, however, are parasitic either upon the outside 

 of the bodies of their hosts (Bdellurd) or in a few cases living 

 in the body-cavity or even being imbedded in the tissues. 



In addition to the ordinary ciliated cells the ectoderm con- 

 tains numerous sensory as well as gland cells. Special 

 glands secrete in most of the groups peculiar rod-like bodies 



