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REGENERATION 



281 



cloaca with its attached respiratory trees, the single 

 gonad, the radial canals of the water-vascular system and 

 the major portion of the dorsal mesentery by which the 

 intestine was suspended. Since this part of the animal 

 continues to give reactions, the student invariably raises 

 the question, "Can Tliyone regenerate the lost parts?" 

 This question was the starting point of the following in- 

 vestigation. The work had not proceeded far when it 

 was discovered that important individual differences 

 occurred, and the question became, "To what extent, or 

 how completely, may these individual variations be re- 

 produced in the process of regeneration?" Curiously 

 enough, the most important differences between individual 

 Thyone involve structures which help to form the radial 

 symmetry of the animal. Consequently the problem has 

 a bearing on the phylogeny as well as the ontogeny of 

 Thyone. 



In general, the results show that regeneration of all lost 

 organs may occur and that there is a decided tendency to 

 even reproduce individual variations. It was found that 

 the Polian vesicles varied greatly in number, size and 

 location. The retractor muscles in a single radius were 

 single or multiple, and for each individual this variation 

 was closely correlated with a corresponding variation in 

 the number of Polian vesicles. Whether one or more 

 Polian vesicles are present, there is a strong tendency for 

 these to occur on the left side of the animal, a fact which 

 undoubtedly has a phylogenetic significance. A more 

 complete statement and a discussion of these results will 

 be given in the following pages. 



General Structure of Thyone 

 Thyone is functionally a bilateral animal. It has ante- 

 rior and posterior ends, dorsal and ventral surfaces, and 

 consequently right and left sides. The external opening 

 of the genital duct is located near the anterior end in the 

 mid-dorsal region. The structure and arrangement of 

 the tentacles is alike on both sides of the animal. Even 



