BY W. H. TWELVETREES AND W. F. PETTERD. 91 



nates the branching network of white spicula, while the 

 fleshy cirri are left dark, and only dimly visible. This 

 method of illnmination very effectively demonstrates the way 

 in which the internal spicular skeleton strengthens the arms. 



Fig. 3. — Transverse section of shell canals of Kraussina 

 Lamarckiana (the typical ribbed kind). This represents 

 nearly the whole thickness of the shell, and shows the 

 oblique direction of the tubular canals x 17. Ordinary light. 



Fig. 4. — Transverse section of shell canals of Kraussina 

 Atkinsoni. Although the shell is somewhat smaller than 

 K. Lamarckiana, the canals are considerably wider. The 

 dark line in the centre is the mesial septum, x 17. Polarised 

 light. 



