236 



METAZOAN PHYLA 



and through the length of the chtellum of the other. Sperm cells are 

 passed out through the vasa deferentia of one worm and flow in two 

 canals formed by apposing grooves on the ventral surfaces of the two 

 animals (Fig. 140 B), to the openings of the seminal receptacles of the 

 other, into which they enter. The same thing occurs on the part of the 

 other animal, and thus a reciprocal exchange of sperm cells takes place. 

 The worms then separate, each having its seminal receptacles filled with 

 sperm cells from the other worm, ready to be used in fertilization when 

 egg laying occurs. At that time a cocoon (Fig. 140 D) is secreted about 

 the clitellum and then slipped forward over the head of the worm. As 

 it passes the openings of the oviducts several egg cells are passed into it ; 

 and as it passes the openings of the seminal receptacles, numerous sperm 



Openings of 

 seminal re. 



Opening of 

 vas deferens 



Seminal groove 

 B 



WWII 



Clitellum 



Fig. 140. — Reproduction in the earthworm. {From Curtis and Guthrie, " Text-book of 

 General Zoology," by the courtesy of John Wiley & Sons, Inc.) A, two worms enclosed in 

 bands of mucus. B, transverse section showing the seminal grooves. C, a sketch to show 

 the path of the seminal fluid from the openings of the vasa deferentia on one worm to the 

 openings of the seminal receptacles on the other. D, the cocoon. 



cells. There is also added an albuminous secretion from skin glands 

 which serves to nourish the developing embryos. As the cocoon leaves 

 the worm the two ends come together and thus is formed an inclosed 

 cavity, within which fertilization takes place. One worm will form many 

 such cocoons. 



The eggs are holoblastic but undergo unequal cleavage; a hollow 

 blastula is formed, and later a gastrula by invagination. All of the egg 

 cells which are contained in the cocoon do not develop into embryos, 

 most of them appearing to serve as nurse cells. Those which do develop 

 produce small worms very similar to the adult, which escape from the 

 cocoon in about two or three weeks. When the young worm first becomes 

 free it lacks a clitellum, that appearing later as the worm acquires sexual 

 maturity. 



Asexual reproduction does not occur in the earthworm. 



