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H. H. NEWMAN 



tiation. In many instances the hydro-enterocoel pouches were 

 well on their way toward being pinched off from the anterior 

 part of the primary archenteron, but no such changes occurred in 

 the secondary archenteron. An interesting case is shown in 

 which an advanced larva has two small supernumerary archen- 

 tera, one near the anterior end and the other near the posterior 

 end (fig. 29). Another case is shown in figure 30, in which two 

 fairly large secondary archentera have grown in at right angles 



37 



Fig. 32 A somewhat rare type of larva in which the primary and secondary 

 archentera, though at right angles to each other, are nearly of equal value. 



Figs. 33 and 35 Types of symmetrical twin larvae in which paired, symmetri- 

 cally placed archentera have fused to form a common 'head end' of the archen- 

 teron. 



Fig. 34 A case of the fusion of anterior ends of two nearly equal archentera 

 asymmetrically placed. 



Fig. 36 An instance of the closing off of a large secondary archenteron in the 

 attempt to regulate back to the 'single' condition. 



Fig. 37 The usual result of fusions like that in figure 34. This is another 

 method of regulation adopted by asymmetrical double larvae. 



