222 



ANIMALS 



wall of the digestive tube). When they are mature they lie 

 free in the body cavity, from which they escape into the water 

 by the breaking of the body of the worm. The sperm and ova 

 escape into the water at the same time. Aftei* fertilization a 

 larva is developed which has no resemblance to an Annelid 

 but is much more like a rotifer. This is called a trochophore 

 larva and is regarded as indicating relationship between the 

 Rotifers and Annelids. From one end of the trochophore larva 



Fig. 125. — The ovum of nereis. Photomicrograph; greatly magnified. 



the body of the worm is developed segment after segment. 

 The development of nereis is, therefore, by metamorphosis. 



502. The earthworm is hermaphroditic, i. e., both sexes are 

 united in one individual, and development is direct. 



503. In the crayfish the reproductive cells are developed in 

 special sack-like organs lying in the thoracic part of the body 

 cavity. The sexes are separate. There is a single ovary, 

 consisting of a pair of lateral lobes connected by a single median 

 lobe. A pair of ducts lead from the ovary to the basal joints 

 of the eleventh appendages where they open to the exterior. 

 The testis of the male is similar in position and composition, 



