Miss 0. M. Rec3 on Actinostola callosa. 



383 



Internal Characters. 



There is a diffuse mesoglceal sphincter muscle extending 

 a considerable way down t lie body-wall. It does not expand 

 suddenly, but is largest in the middle region and tapers off 

 very gradually as it is traced downwards. It docs not 

 occupy the entire width of the column-wall, but lies through- 

 out its course nearer the endodermal surface than the ecto- 

 dermal. Ou its inner surface its cavities pass directly into 

 the ordinary longitudinal muscle of the mesoglcea of the 

 column-wall. The sphincter muscle in cross-section appears 

 as a network of fibrilhe, and the spaces between the fibrous 

 meshes appear empty. [In spirit.] 



In the living animal these spaces are filled with the proto- 

 plasm and the nuclei of the muscular corpuscles. There is 

 also a tendency for these closely packed cavities to be 

 arranged in longitudinal bands separated from one another 

 by strands of nearly homogeneous mesoglcea, recalling the 

 arrangement described by Hertwig and McMurrich. 



Fig-. 1. 



Falkland specimen of Actinostola callosa. 



The number of septa is large. They amount to ninetv-six 

 pairs, which are distributed in five cycles. Twenty-four 

 pairs reach the stomatodasum to a less extent than the other 

 twelve. In addition to these, there is another cycle of twenty- 

 four imperfect pairs — this is the fourth cycle; while the 

 fifth cycle of forty-eight pairs consists of imperfect mesen- 

 teries which are very small and project only a short distance 

 beyond the endoderm. They bear neither reproductive 

 organs nor mesenterial filaments, but of each pair one 

 mesentery is more highly developed than its fellow. This 

 specimen is a male, and the tors are borne on all the 



