AND RELATED GENERA, SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 55 



form normal pairs with neighbouring mesenteries of the first order. At this stage, 

 therefore, the genera have 10 pairs of mesenteries, 2 of which, lying medially 

 opposite to each other, are directive mesenteries, whilst the other 8 are formed in 

 pairs with typically arranged longitudinal muscles. In the 8 new endoccels arisen 

 through the growth of the mesenteries of the second order, there is a further develop- 

 ment of 8 normal pairs of the third order, so that at this stage we have 10 (6 + 4) + 8 

 = 18 pairs of mesenteries. Thereafter the development proceeds in a more normal 

 manner, the mesenteries of the fourth and fifth order in Porponia not developing in 

 the endocoels of the third order, but on both sides of a pair of the third order ; in an 

 exocoel in relation to the mesenteries of the third order, but in an original endoccel of 

 the second order. I have endeavoured previously (1897) to show that the develop- 

 ment proceeds in this way in Endoccelactis, as with fairly great certainty I considered 

 it possible, partly from the unequal development of the mesenteries at the basis of the 

 column, to distinguish the first 6 pairs of mesenteries (1,1, etc.) from the 4 pairs of the 

 second order, and thence from the obviously great development of these 10 pairs in 

 comparison with the others to distinguish them clearly from the subsequent orders of 

 mesenteries. In Porponia I had admittedly not been able to study the development 

 of the mesenteries so clearly as Endoccelactis, as the difference between the mesenteries 

 of the said orders were not so distinct as in this latter form, owing to the fact that 

 in Porponia several more complete mesenteries occur than in Endoccelactis. The 

 arrangement of tentacles in Porponia, as also the whole arrangement of mesenteries, 

 indicates, however, that in this genus the mesenteries arise in the same manner, and that 

 also after reaching a stage with 6 normally placed pairs of mesenteries, a development 

 of mesenteries in the endocoels has taken place. The groups of tentacles of the first and 

 second orders, which arise near the directive mesenteries 1, 2, 1 (dt.), 2, 1, and those 

 which lie in the transverse plane 1,2, 1, are inside the tentacles of the corresponding 

 order which stand at the other 4 pairs of stronger mesenteries, a condition that is to 

 some extent indicated on the schematic figure, but which in reality is considerably 

 greater than the figure shows. This indicates that we have to arrange the first 6 pairs 

 of mesenteries by this plane. With regard to the arrangement of the tentacles other- 

 wise, this is in the main the same in the two genera, great displacements occurring in 

 the cycles with the development of certain mesenteries in the endoccels, wherewith, so 

 to speak, a doubling of the tentacles in the lateral endocoels of the first and second 

 orders arises. Above all, the arrangement of the 28 innermost tentacles is the same 

 in both genera, as the figure shows. It is characteristic of both genera, therefore, that 

 all the mesenteries of the second and third orders develop in the endoccels, and that 

 in consequence great displacements in the position of the tentacles take place. 



Nevertheless, there are a number of differences in the structure of the two genera. 

 In a number of less important characters, such as the form of the body — in Porponia 

 beaker-like, in Halcurias more cylindric — in the presence of only one oesophageal 

 groove in Halcurias, whilst Porponia has two, there are certainly differences between 



