REPRODUCTION 



Reproduction of planarians is either sexual or asexual. Sexual repro- 

 duction usually involves copulation of two individuals during which the 

 sperm of one partner are deposited in the copulatory bursa of the other 

 (autofecundation has been observed only in Cura foremanii among the 

 American planarians). From the bursa the sperm are expelled through the 

 bursal duct, enter the oviducts, and are finally stored in the seminal 

 receptacles. The ova are fertilized as they pass through the receptacles 

 on their way to the atrium where they accumulate. They are deposited in 

 a cocoon, a rather large (diameter 0.5 to over 3 mm) spherical or 

 ellipsoidal capsule with a shell of horny consistency, attached to the 

 substrate by a gelatinous secretion or, more rarely, by a thin stalk 

 (Fig. 4). 



A B 



Fig. 4. Egg capsules of planarians. A, unstalked cocoon; 

 B, stalked cocoon. 



Each capsule contains several (2-20, sometimes more) zygotes and 

 thousands of yolk cells which serve as food for the developing embryos. 

 When the young hatch from the cocoon one to several weeks after its 

 deposition, they have more or less the shape of the adults, without 

 passing through a larval stage and metamorphosis. They lack, however, 

 any trace of the reproductive system which will develop only when they 

 are almost fully grown. 



Asexual reproduction is by fission or by fragmentation. Fission occurs 

 usually at a postpharyngeal level and consists of a tearing apart of the 

 posterior portion of the body. Both parts then regenerate the missing 

 organs, the anterior part a new tail and the posterior part a new head, 

 pharynx, etc. In fragmentation the body breaks up into several smaller 

 portions each of which then encysts, i.e., acquires a rounded shape and 

 encloses itself in a layer of transparent slime which usually adheres to 

 the substrate. After a certain rest period small planarians hatch from 

 the cysts, resembling the young hatched from an egg capsule. Asexual 

 reproduction is very common in some species, and certain populations have 

 been cultured through several generations without developing sexual 

 structures . 



