CHAPTER IV 



AGE DIFFERENCES IN SUSCEPTIBILITY IN THE LOWER 



ANIMALS 



THE EXPERIMENTAL MATERIAL 



Three species of fresh-water planarians, Planaria dorotocephala, 

 P. maculata, and P. velata, have constituted the chief material for 

 the more extended investigations. P. dorotocephala is found in 

 great abundance in various parts of the United States, chiefly in 

 springs and the streams issuing from them. In nature the animals 

 usually attain a length of twenty to twenty-five millimeters, but 

 in the laboratory with abundant food may reach double that 

 length. 



The body, like that of most turbellaria, is dorso-ventrally 

 flattened ; the body- wall consists of a one-layered ciliated ectoderm 

 beneath which lie longitudinal and transverse muscle layers and 

 in the spaces between the internal organs a parenchymal tissue. 

 A pigment layer beneath the dorsal ectoderm gives the dorsal sur- 

 face a deep-brown color, the ventral surface being much less deeply 

 pigmented. The chief features of the internal anatomy are indi- 

 cated in Fig. 6. The central nervous system consists of a pair of 

 cephalic ganglia beneath the eyes and two longitudinal cords (ns) 

 which give off branches and are connected by commissures. The 

 chief sense-organs are the eyes, consisting of pigment cups con- 

 taining sensory cells and the lateral pointed cephalic lobes, which 

 are organs of chemical sense. The margins of the head and body 

 are also sensitive tactile organs. 



The mouth (m) lies ventrally in the middle of the body and opens 

 into a pharyngeal pouch containing a tubular pharynx (ph). At 

 its anterior end the pharynx opens into the alimentary tract which 

 consists of three main branches (a/) and many secondary branches. 

 A diffuse branching excretory system is also present, but not shown 

 in the figure. Under the usual conditions the animals do not 

 become sexually mature, and sexual organs if present at all do 

 not develop beyond very early stages. 



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