586 JAMES F. GEMMILL. 



PLATE II. 



{For lettering see p. 584.) 



Fig. 12. — Normal bipinnaria of Luidia of same age as the twin bipinnariae described. 

 It will be noted that the preoral and postoral ciliated bands are completely developed 

 and that in the ahmentary canal, buccal cavity, oesophagus, stomach, and rectum can 

 all be made out. There is a pair of enterocoeles, the left one being provided with a 

 hydropore. 



Figs. 13-21. — A series of double monster bipinnarise about six days old, showing 

 different kinds and degrees of duplicity. Here we must pay attention to a number of 

 details, e.g. the preoral and postoral bands and the surface areas which they mark out, 

 also the mouth and buccal cavity, the oesophagus, stomach, and rectum, the right and left 

 enterocoelic cavities and the hydropore and hydroporic canal. In general the doubling 

 is greater internally than externally, the enterocoeles and various parts of the food- canal 

 being sometimes in two sets without a corresponding degree of division being exhibited 

 on the surface of the body. 



Fig. 13 is an example of parallel lateral union, as seen from the dorsal aspect. The 

 whole of the food-canal is doubled, the mouth and anus in both cases looking in the 

 same direction. Each food-canal has its own pair of enterocoeles, the left one in both 

 instances developmg a liydropore and hydroporic canal. The whole bipuinaria is broader 

 than normal ; there is a single preoral ciliated band enclosing the widened frontal field ; 

 the postoral ciliated band is also single, but shows a deep backwardly directed sinus on 

 the ventral side marking off the twin anal fields from one another. 



In Fig. 14 one set of structures is much better developed than its neighbour, the 

 latter not showing mouth or anus, though provided with oesophagus, stomach, intestine, 

 and a i^air of enterocoeles. Only one of the preoral ciliated bands is properly differen- 

 tiated, viz. that in connection with the frontal end of the better developed twin. Tlie 

 view is from the ventral aspect. 



Fig. 15. — A double monster bipinnaria m which one of the twins is smaller than, and 

 set at right angles to, the other. The smaller has no mouth and its frontal field is deficient 

 in size. The postoral ciliated bands run into one another. 



