340 H. H. NEWMAN 



fairly deep invaginations of the ectoderm destined for the stomo- 

 daeum. One might expect such a larva as this to display evi- 

 dences, in connection with its locomotion, of discord between 

 the two components, but this was not the case, for the larva 

 went ahead at a rapid rate revolving on the long axis and always 

 keeping that end in advance at which the smaller, evidently 

 secondary, archenteron opened, so that this second individual 

 proceeded backward with its blastopore ahead and its own head 

 end behind. This twin larva was kept under observation for 

 over a week and little further advance was noted. No distinct 

 ciliated bands were formed, but small vesicles of mesenchyme 

 about the anterior end of the posterior archenteron (lacking in 

 the anterior archenteron) indicated an attempt to form the 

 hydro-enterocoel cavities. The presence of the secondary arch- 

 enteron evidently inhibited the full consummation of this process. 

 Another unusually large, active larva of somewhat similar type, 

 but with two secondary archentera, is shown in figure 39. In 

 this case no hydro-enterocoel pouches were produced, although 

 in normal larvae of the same age and size an advanced stage of 

 coelom differentiation had been reached. The various types 

 of twin larvae live for a long time. In figures 40, 41, and 42 are 

 shown three larvae drawn at twenty, twenty-four, and eighteen 

 days, respectively. They all show considerable differentiation 

 of the alimentary tract into oesophagus, stomach, and intestine. 

 Figure 40 probably arose from a larva similar to that shown in 

 figure 12. The left-hand archenteron has undergone the flexure 

 of the stomach characteristic of normal larvae. The right-hand 

 archenteron, however, is somewhat less advanced. The larva 

 shown in figure 41 presents an intricate appearance. The two 

 archentera show advanced differentiation and are mirror-image 

 duplicates so far as differentiation is concerned, both showing 

 the same flexures and the same degree of differentiation of the 

 oesophagus, stomach, and intestine. Figure 42 shows a rare 

 case of an advanced larva of the type shown in figure 33. Here 

 the anterior, fused region is relatively inhibited, while the stomach 

 and intestine are clearly differentiated. 



