DOMINANCE AND PHYSIOLOGICAL ISOLATION 323 



entially inhibited heads are fed, the length attained before fission is 

 less than in animals with normal heads and decreases from teratoph- 

 thalmic, through teratomorphic, to anophthalmia head forms (p. 178). 

 Acephalic forms often divide near, or even anterior to, the middle when 

 stimulated to contraction, though only 3 or 4 mm. in length, dominance 

 being almost absent. 



STENOSTOMUM 



In the rhabdocoel families Stenostomidae and Microstomidae morpho- 

 logical differentiation of zooids precedes fission; and since increase in 

 length is more rapid than differentiation and separation of zooids in well- 

 fed, rapidly growing animals, new zooids develop before separation of 

 earlier generations; consequently, long chains of zooids in various stages 

 of development result (see Fig. 10, p. 27). Each zooid in the chain under- 

 goes division when it attains a certain length, and in Stenostommn the 

 length attained by any zooid before a new fission zone appears in its 

 posterior region increases with advance in development of its head, but 

 the length of a zooid when it first becomes distinguishable is approxi- 

 mately the same at different levels of the chain under uniform conditions. 

 Apparently the length of body that a particular head can dominate in- 

 creases as development of that head progresses. This relation is evident 

 in Figure 10. 



With each new zooid new cephalic ganglia develop and reorganization 

 of the nerve cords posterior to each new head must result. It is probable 

 that the progressive extension of dominance posteriorly with advance in 

 head development is associated with progress posteriorly of this reorgani- 

 zation. In animals living at low temperatures with little food, conse- 

 quently with slow increase in length, single zooids may become as long as 

 well-fed chains at higher temperatures. 



Stenostomum is a protandric hermaphrodite. When a chain attains the 

 male stage, the zooids already present in the chain develop and separate, 

 but there is no further development of new zooids. When the individuals 

 attain the later female stage, they cease to feed and the pharynx degen- 

 erates; but they continue to elongate until they are as long as many of 

 the agamic chains, but without development of new zooids. Apparently 

 increase in range of dominance keeps pace with the elongation; it is less 

 rapid than in well-fed chains. After section of the elongated females at 

 postcephalic levels the posterior piece remains acephalic' In this respect 



9 These observations and experiments are chiefly with Stenostomum grande, but other species 

 show similar relations of zooids in the chains. Sexual individuals of other species have not 

 been obtained by the writer. See Child, 1902; Van Cleave, 1929. 



