128 G Canadian Arctic Expedition, 1913-1918 



In a transverse section twelve complete mesenteries and eight incomplete 

 ones can be seen (PL XXV, fig. 1;' pi. XXVI, fig. 8). They are unequal in the 

 two sides of the body, due probably to contraction. Their longitudinal muscles 

 in the upper part are large, restricted, somewhat reniform or crescent-shape* 1 in 



Fig. 16. Bicidiopsis aretica Verrill. Type. From a contracted Hudson bay specimen. 



sections, thick in the middle region, thinning out towards each edge, and re- 

 curved (PL XXV, fig. la). At the base of the stomodseum, they are thinner 

 and not recurved (PL XXVI, fig. 8). All the perfect mesenteries bear con- 

 voluted mesenterial filaments (PL XXV, figs. If-lh). The mesenteries all 

 reach nearly to the central pore in the posterior end, and end abruptly (figs. 

 Ij, Ik). The sphincter muscle (fig. le) is thin and diffuse. The column-walls 

 are rather thin, the ectoderm layer (g), being thick, glandular, and soft exter- 

 nally (fig. li). The mesoglcea (1) shows five, six, or more, narrow parallel lines 

 for the circular muscles (h 1 )- 



This species is evidently very closely related to Bicidiopsis quinquecapitata 

 (McMurrich), referred to Peachia by him. He described it as from Nanoose 

 bay, Vancouver island, in 15-20 fathoms. (Proceedings of the Zoological 

 Society of London, 1913, p. 963, pi. 98, figs. 1-4). 



His species has the same number and the same arrangement of mesenteries, 

 and the retractor muscles of the mesenteries have nearly the same form. Its 

 conchula is three-lobed, the median lobe is small and bears a median papilla; 

 each side lobe bears two papillae, as in my species. Thus the conchula has 

 essentially the same structure as in the Hudson bay specimens. 



The agreement is so close that it is possible that the two belong to the same 

 species, but additional specimens should be studied before this can be decided 

 definitely. 



Prof. McMurrich, in the same article, (p. 967, PL 98, figs. 5-7) described 

 Bicidium wquorece, a iiew species found parasitic on the bell of Mquorea forskalli, 

 and suggested that it may be the larval form of the preceding species. It has 

 a rudimentary conchula, and lacks the five papillae. It had only 6 pairs of mesen- 

 teries, all of them perfect. 



Its mesenterial muscular pennons were much thinner than in the preceding 

 species. It seems to me unlikely to be the young of the latter. It was nearly 

 as large. I believe it should be called Siphonactinia cequorece. 



