58 MORPHOLOGY OF IXYKKTKBKATE TYPES 



Specialized organs of circulation and respiration are absent. 



Excretory system. The excretory system consists of two 

 longitudinal canals, running the whole length of the body and 

 anastomosing anteriorly; they are the so-called collecting canals. 

 The capillary canals, present in great numbers and each closed 

 distally by a. flame cell open into the collecting canal. The col- 

 lecting canals open to the outside on the dorsal surface by means 

 of short tubes. There are altogether four pairs of excretory pores. 



Nervous system. The nervous system consists of a brain oc- 

 cupying the anterior end of the animal and of two longitudinal 

 nerves extending to the posterior end of the animal. The brain 

 is composed of two lobes connected with each other by a trans- 

 verse commissure and supplies with nerves the tentacles and the 

 median lobe as well as the eyes. The longitudinal nerves are 

 connected with each other by numerous commissures and give 

 off numerous branches which innervate the entire body. 



Reproductive system. The great majority of Turbell- 

 arians including D. lacteum are hermaphrodites. The male and 

 female genital organs open into a genital cloaca communicating 

 with the outside by means of a common genital opening which is 

 situated on the ventral surface at some distance behind the 

 mouth. The male genital organs consist of numerous tcstes be- 

 ginning shortly behind the ovaries and extending to or even 

 beyond the region of the common genital opening, of two rather 

 short vasa defcrentia which open into a penis pouch, a muscular 

 penis and a group of penis glands (prostata). There seem to be 

 no efferent ducts leading from the testes to the vasa deferentia; 

 the spermatozoa reach the latter through the interstices of the 

 parenchyma. As already stated, the penis opens into the 

 genital cloaca and lies usually invaginated in the penis pouch. 

 During copulation the penis becomes evaginated and protrudes 

 through the common genital opening. 



The female genital organs consist of three groups of organs not 

 directly connected with each other, but each with a separate 

 opening leading into the genital cloaca. The first group consists 



