372 Animal Biology 



and longitudinally, to the posterior part of the body. Several transverse 

 nerves connect the two ventral nerve cords (Fig. 178) Ciliated pits on 

 either side of the head contain special sensory cells. 



Planaria illustrates axiate organization. It has (1) a primary antero- 

 posterior axis and (2) a secondary ventrodorsal axis. When the primary 

 axis is considered, we find an axial gradient of metabolic activity which 

 decreases as we proceed from the anterior end toward the posterior end. 

 In other words, the more highly active areas of the anterior regions, 

 because of their activity, assume a control over the less active posterior 

 regions as we progress in sequence from anterior to posterior. When 

 the secondary ventrodorsal axis is considered, we find the greatest activ- 

 ity in the ventral side. Activity gradually decreases as we go toward 

 the dorsal side. When these two axes are considered at the same time, 

 the region of greatest activity is located at the anterior end near the 

 ventral side. Planaria also has bilateral symmetry in which the right 

 and left halves are similar to each other. Because of the axial gradient, 

 Planaria may be cut transversely into pieces and each segment under 

 proper conditions will regenerate and eventually assume its normal ac- 

 tivities with new centers of control established in each piece (Fig. 28). 



Planaria responds to a variety of stimuli. One pair of dorsal eye spots 

 are sensitive to light. One pair of lateral tactile olfactogustatory auricles 

 connected with the brain are affected by touch and certain chemicals (L. 

 tactus, touch; L. olfacio, smell; L. gustus, taste). 



Reproduction. — Both male and female sex organs are present in the 

 same Planaria; thus it is monecious or hermaphroditic (mo -ne' si us; 

 her maf ro -dit' ik) (Gr. monos, one; eikos, house) (Gr. hermaphroditos, 

 combining both sexes). However, self-fertilization probably does not 

 occur, but cross fertilization is used (Fig. 179). The male organs include 

 numerous spherical testes scattered throughout the body. The testes are 

 connected by sperm ducts or vasa deferentia (singular, vas deferens). 

 Each of the pair of large sperm ducts enlarges at its posterior end into a 

 seminal vesicle (L. semen, seed; vesica, bladder) . The latter connect with 

 the muscular copulatory penis. The sperm produced by the testes are col- 

 lected by the vasa deferentia and carried to the seminal vesicles until 

 transferred by the penis to the opposite animal during copulation. The 

 penis projects into the genital chamber located posterior to the pharynx. 

 The sperm are transferred by the penis to the seminal receptacle (copu- 

 latory sac) of the opposite animal during copulation. The saclike semi- 

 nal receptacle is connected by a tube with the genital chamber. 



