FLAT WORM (pLANARIA) 329 



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 vesiclies, 

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

 opens into the common cavity called the genital atrium or genital 

 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 

 by way of the vagina. There are numerous yolk glayids joining each 

 oviduct along its length; a glandular structure of questionable func- 

 tion, in the form of a blind tube with an inflated end, is connected 

 with the genital atrium. It has been suggested that the fertilized 

 eggs accumulate and are retained here for a time. The system is 

 notably quite elaborate, and it is found generally that the flatworms 

 have a highly specialized reproductive system. 



The planarian worms and the representatives of this phylum pos- 

 sess no skeletal system, no respiratory system (it breathes through 

 its skin); no coelom or body cavity; and no circulatory system; 



