AND RELATED GENERA, SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 57 



the genera, but also others more important exist. In the two genera the tentacles are 

 formed in a different manner, having the usual appearance in Halcurias, whilst they 

 are greatly thickened in Porponia on the outer side, so that it looks as if the thick 

 mesoglofta of the body-wall extended in the form of bridge-like outshoots into the 

 tentacles. An important distinction between Porponia and Halcurias is also seen in 

 the development of the mesenteries. Whilst Halcurias has only 10 pairs of stronger 

 mesenteries — the mesenteries of the third order being thus but feebly developed, and 

 that only in the most distal end — there is a much larger number of stronger mesenteries 

 in Porponia, which are connected with the oesophagus in its whole length or the 

 greater part of this. Very characteristic also is the fact that the development of the 

 mesenteries, which appear after the mesenteries of the third order, is different in the 

 two genera. In Halcurias the mesenteries of the fourth order arise normally, so that 

 the two mesenteries in the same pair appear simultaneously ; the development then 

 ceases, though it is possible that in some cases mesenteries of a fifth order may be laid 

 down. In Porponia, on the other hand, the development has taken a different line, 

 and comes to resemble that of the later mesenteries in Actinostola and Stomphia. The 

 mesenteries of the fourth order, namely, do not appear at the same time ; on the one 

 hand, the one mesentery is developed much earlier than its pair ; on the other, the 

 development of the mesenteries of the fourth order is delayed in certain regularly 

 arranged exocoels, so that here only one unpaired mesentery instead of a pair arises. 

 This displacement of the mesenterial appearance has had the result that an unpaired 

 mesentery of the fifth order arises on the side where the strongest and perfect mesentery 

 of the fourth order lies. In fact, the development of the mesenteries of the fourth and 

 following orders in Porponia comes under the same law as I have indicated for the 

 development of later mesenteries in Actinostolidse, and which means that the develop- 

 ment of a stronger mesentery in a previous order leads to an earlier development 

 of the mesenteries of the subsequent orders in the area which lies nearest this 

 stronger mesentery. Finally, it has to be noted that irregularities not rarely arise in a 

 quadrant of Porponia which shows a somewhat larger number of mesenteries than the 

 normal. If we indicate the mesenteries with numbers, those of the first order with 1, 

 of the second with 2, and so on, the irregularity in the arrangement of the mesenteries 

 on each side of the directive plane can be seen from the following scheme for the two 

 genera (dm. = directive mesenteries) : — 

 Halcurias {Endocadactis). 



1 (dm.) 1, 4, 4, 3, 3, 4, 4, 2, 2, 4, 4, 3, 3, 4, 4, 1, 1, 4, 4, 3, 3, 4, 4, 2, 2, 4, 4, 3, 



3, 4, 4, 1, 1 (dm.) = 34. 

 Porponia. 



1 (dm.) 1, 5, 4, 4, 3, 3, 4, 2, 2, 4, 3, 3, 4, 4, 5, 1, 1, 5, 4, 4, 3, 3, 4, 2, 2, 4, 3, 3, 



4, 4, 5, 1, 1 (dm.) = 34. 



The arrangement of the tentacles is also somewhat different in the two genera (see 

 figures). This difference is due entirely to the different arrangement of the mesenteries 

 TRANS. ROY. SOC. EDIN., VOL. L. PART I. (NO. 4). 8 



