THE MESEN'TERIES IN URTICINA CRASSICORNTS. 



Text-figure 5. 



4 13 9 



455 



Pig. 1. 



Pig. 2. 

 Fig. 3. 



Explanation of Text-piguees 1-5. 



Transverse section (diagrammatic) of late larva of Urticina crassicomis 

 to illustrate the arrangement of the 8 JSdtvardsia mesenteries. 



Similar transverse section, fourteen da3's after fixation, to illustrate the early 

 12 mesenteried stage. 



Diagrammatic transverse section of early grovi'th stage in which an addi- 

 tional pair of mesenteries (Nns. 7, 8) has appeared in each lateral and 

 ventral primary exocoele. 



Pig. 4. Similar section of later growth stage in which the additional mesenteries 

 seen in fig. 3 have become mesenteries of the 1st Order, and a further pair 

 (No. 9) belonging to the same (i.e., the 2nd) cycle has appeared in each 

 dorsal primary exocoele. The latter w-ill ultimatelj' take rank as a 

 mesenteric pair of the Ilnd Order. 



Pig. 5. Similar section of still later stage in which {a) mesenteric pairs 10, 11 

 (3rd cycle mesenteries) have appeared and are growing forward to take rank 

 with 9 as mesenteries of the Ilnd Order, while (6) new mesenteric pairs of 

 the Illrd Order (12-16) are Ijeing formed (see explanation in text) in all 

 the exoccfiles. 



arrangement of the muscle banners on the 12 mesenteries now 

 present is such as to group them into the six primary pairs 

 characteristic of hexactinian symmetry. This agrees with the 

 results furnished by the latest stages investigated by Appellof 

 (1. p. 82). 



