NBPHRJDIA OF THE ACTINOTEOCHA LARVA. Ill 



already alluded bo above. The remainder of the hood-cavity 

 is for the most part occupied by a more or less dense mass of 

 delicate fibres, with occasional nucleated mesenchymatous 

 cells. The presence of the fibres may be to some extent due 

 to the coagulating action of the fixative. 



As already stated by Ikeda, with regard to Phoronis 

 ijimai, neither dorsal vessel nor dorsal mesentery exists in 

 the pre-septal " collar " region of the larva. It is difficult, 

 indeed, to see how these structures, described by Masterman, 

 could be present, since the pre-septal cavity is itself a part of 

 the blood-system, containing blood-corpuscles, and giving 

 rise in later stages to the lophophoral blood-vessel. 



There remains to describe the true pre-septal coelom. 

 Strangely enough Ikeda is the first and only author who 

 has given a correct account of this cavity (9). "The adult 

 collar cavity, or the supra-septal cavity,^' says Ikeda, "is 

 already formed in the fully developed larva of every type as 

 a ring space running along the inner side of the tentacular 

 circle and above the septum." He further shows that this 

 ring-like rudiment of the adult pre-septal ccelom sends out a 

 caecum into the base of each tentacle. Ikeda gives no 

 details as to the origin of the pre-septal coelom, nor have I 

 been able to follow it for certain myself. It makes its 

 appearance somewhat late in larval life, when the venti-al sac 

 is already well formed, as a split between two layers of a 

 narrow band of mesoblast adhering closely to the body-wall 

 just above the septum and just below the tentacles. So far 

 as I have been able to make out the ring is never completed 

 dorsally. An advanced larva shows a pre-septal coelom of 

 considerable size, which rims round the base of the tentacles, 

 sending out a blind process into each (tigs. 2 and 10). It 

 terminates dorsally in two horns, which run forwards for 

 some little space on either side of the middle dorsal line 

 towards the ganglion (figs. 2 and 13). 



We may now attempt to give in a few words a clear 

 description of the cavities in the Actinotrocha larva. 



Behind tlie circle of tentacles there is a single trunk 



