168 TEXTBOOK OF ZOOLOGY 



The cellular walls of the tubules as well as the flame cells arise in 

 the mesoderm. Under strict definition, some authors object to calling 

 this arrangement a system. 



Another mesoderm organization is the muscular system. It is com- 

 posed of an outer circular layer just under the epidermis; an outer 

 longitudinal layer just medial to the circular layer; oblique bundles 

 of fibers ; and at the medial margin of the mesoderm is another ir- 

 regular, internal, longitudinal layer just medial to a circular layer. 

 By the alternate activity of these layers the animal is capable of 

 great extension and contraction. 



Another advanced development is the "ladderlike" nervous 

 system which consists of two contiguous lobes of nerve cells just 

 ventral to the eyespots, two ventrolateral longitudinal nerve cords, 

 transverse commissures, branch nerves, and sensory end areas of the 

 epidermis. The double ganglion at the anterior is the central portion 

 of the system. It is known as the cephalic ganglion and gives 

 branches to sensory areas of the head, auricles, etc., besides joining 

 the longitudinal nerve cords. The transverse commissures connect 

 the two longitudinal cords at from 15 to 20 points like the rungs of 

 a ladder. At each point where a transverse commissure meets a 

 longitudinal cord, is a small ganglion composed of a few nerve cell 

 bodies. The branch nerves extend to the surrounding tissue from 

 these points. 



The reproductive system is fairly well developed in most species 

 except P. dorotocephala which rarely develops sexual organs. Its 

 reproduction is entirely by asexual fission. The sexual reproduction 

 of other planarians is hermaphroditic, which is rather characteristic 

 of sedentary animals. The male organs consist of numerous globular 

 testes located in the parenchyma through most of the length of the 

 body. Vasa efferentia are slender, thin-walled ducts leading from 

 the testes to two larger, longitudinal ducts, the vasa deferentia. 

 These in turn lead posteriorly, enlarge to become seminal vesicles, 

 and converge to form the penis or cirrus, the copulatory organ. This 

 opens into the common cavity called the genital atrium or geiiital 

 cloaca, which opens externally at the genital pore. Some authors 

 describe glands which pour a seminal fluid into the system. The 

 female organs in the same animal consist of two ovaries located well 

 toward the anterior, a tubular oviduct leading posteriorly from each 

 to join the genital atrium at a common point near its posterior end 



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