THE THREE-LAYERED ANIMALS 



165 



Of the various systems, the planarian 

 reproductive system shows the greatest ad- 

 vancement over that of the coelenterates 

 (Fig. 9-6). In the sexually mature worm, 

 male and female reproductive systems are 

 present in each individual, a condition 

 known as monoecious (hermaphroditic). 

 Both ovaries and testes develop from the 

 cells of the parenchyma. The numerous 

 testes are rounded bodies which lie along 

 botli sides of the body. They give rise to the 

 spermatozoa, or sperm cells, which are con- 

 veyed through small ducts, the vasa effer- 

 entia, to a larger tube, the vas deferens, or 

 sperm duct, running the length of the body 

 on each side. The two seminal vesicles ter- 

 minate in a pear-shaped copulatory organ 

 or penis. At rest the copulatory organ opens 

 into the genital atrium, which leads into 

 the genital pore, the external opening 

 through which the penis is thrust during 

 copulation. 



The two ovaries of the female reproduc- 

 tive system are found near the anterior end 

 of the body; these produce the ova. The 

 yolk glands, which give rise to the yolk and 

 shell of the egg, are found along the ovi- 

 ducts. The two oviducts lie parallel to the 

 nerve cords and join before entering the 

 atrium. The seminal receptacle, a sac for 

 storing sperm, also opens into the atrium, 

 very near the external opening. The genital 

 atrium, therefore, receives the openings of 

 both the female and the male organs. 



At the start of copulation the ventral sur- 

 faces of the two animals come together, so 

 that the openings of the genital atria are 

 opposite one another. The penis of each is 

 extended into the genital opening of the 

 other and the sperm cells are exchanged. 

 At the time of copulation the ova are also 

 ripe. To prevent self-fertilization, the geni- 

 tal area has been elaborated. The penis, 

 when extruded and dilated, completely fills 

 the atrium and thus blocks the openings 

 into the oviducts, so that neither can the 

 ova escape nor sperm cells enter the ovi- 

 duct, but sperm can be deposited into the 



testis 

 oviduct. 



yolk qland 



spcrro duct. 



soronol rcceptacla 

 copulatory onqao 



qenital atriuo)_ 



qeoital pore 



seminal vesicle 



Fig. 9-6. Planaria in dorsal view and cross-section show- 

 ing the reproductive system. Upper right indicates 

 side view of how cut was made. 



seminal receptacles. At the completion of 

 copulation the penes are withdrawn and the 

 sperm cells then are able to enter the ovi- 

 duct. The ova are fertilized in the oviduct 

 and, as they move down toward the atrium, 

 the products of the yolk glands are dis- 

 charged. The mature fertilized egg is re- 

 leased from the atrium through the genital 

 pore, and the capsule-like shell becomes 

 attached by a stalk to submerged objects. 

 The egg cases may undergo a rest period 

 before growing into young planaria. 



Planaria also reproduces asexually by 

 means of transverse fission. Indeed, this is 

 much the more common method of repro- 



