CYNTHIA ECHINATA. 95 



[The test of this species is represented by fig. 10 on 

 Plate XXX.] 



The mantle (PI. XXXII, fig. 8) is firmly attached 

 to the test, particularly in the region of the tubes, and 

 the tubes themselves cannot be withdrawn without 

 laceration ; it is rather thin, tough, and transparent ; 

 the muscular fibres are delicate and regularly arranged, 

 somewhat apart; those radiating from the tubes being 

 the best displayed; the inner surface seems to be devoid 

 of the usual soft nodules. The tentacular filaments 

 (fig. 59), about sixteen in number, are alternately large 

 and small, rather long and slender, and incompletely 

 tri-pinnate. 



The branchial sac is delicate with five folds on each 



FIG. 59. A tentacular filament of Cynthia echinata. Highly magnified. 



side, four of which are well developed, and one, that 

 next the endostyle, is narrow. The blood-channels are 

 peculiarly arranged. Stout longitudinal vessels pass 

 with much regularity from end to end of the organ, 

 and, but for this arrangement, would be taken for 

 primary vessels ; and other small vessels having the 

 appeai'ance of secondary vessels, divided by stomata, 

 are placed transversely between the large ones ; the 

 stomata are long with rounded extremities. The small 

 vessels in this case must conduct the blood from the 

 dorsal to the ventral channel and consequently act as 

 primary vessels ; while at the same time the chief 

 aeration must take place through their walls, for here 

 the blood is the most minutely divided. Nevertheless 

 from their anatomical connections they should perhaps 



