66 CERIANTHARIA. 



mesenteries of the ist and and cycles is just a simple thread which exactly answers to the craspe- 

 donemes of the ciliated tracts, but with this difference naturally, that no ciliated streaks are developed 

 on it and that the ascending and descending limbs of the filament consist of a cnido-glandular tract and 

 not of a median streak. The descending and ascending limbs of the cnido-glandular tract are separated 

 by an endoderm portion. The cnido-glandular tract on the metamesenteries of the first and second 

 cycles in P. solitarius consists then, we may say, simply of a craspedoneme. From this starting 

 point a number of intermediate stages are found, leading up to the very wavy cnido-glandular tract 

 of the metamesenteries of the 3rd and 4th cycles. For no more than between the craspedonemes of 

 the ciliated tract region and its bunches of craspedonemes, is there any essential difference between 

 the simple craspedoneme of the cnido-glandular tract and the much coiled filament of that region. 

 The simple craspedoneme of the ciliated tract region has its counterpart in the simple craspedoneme 

 of the cnido-glandular tract, the bunches of tightly compressed craspedonemes in the ciliated tract 

 region are represented in the cnido-glandular tract by the very wavy coils on the metamesenteries of 

 the third and fourth cycles in a number of species. 



The Craspedion region (Filamentchenregion Carlgren 1912). 



The Craspedion is the name I have given above to that part of the filament, where the 

 course of the median streak (spirocyst-glandular tract) is simple without ciliated tracts. 

 They occur on the one hand in species with "acontia" (consequently in Arachnactis lobiancoi, albida, 

 Arachnanthus oligopodus and sarst) on mesenteries of the first and second cycles in an oral direction 

 from the "acontia", between these and the ciliated tract region, and on the other hand they are 

 found on mesenteries, which are very long, have cnido-glandular tracts running to no great distance 

 from the stomatodaeum, which is the case with protomesenteries 2 and the metamesenteries of the 

 first and second cycles in C. lloydii, vogti and C. americanus, with the metamesenteries of the 2nd 

 cycle in P. maua, and with all mesenteries but the directive in Botrucnidifer norvcgicus. In C. lloydii 

 and vogti they take up the greatest part of the length of the mesenteries below the cnido-glandular 

 tract, and the same thing happens on the metamesenteries of the first cycle in Ceriantheopsis americanus. 

 In B. norvcgicus they stretch between the cnido-glandular tract and the botrucnidae at the aboral 

 extremity of the mesenteries. But the Craspedion region is very rudimentary in B. nor-vegicus on the 

 metamesenteries of the third and fourth cycles. 



Form and structure of this region alike vary to some extent among the different Ceriautharia. 

 As regards form at least, this is connected with the varying appearance of the filaments. In structure 

 the craspedion mostly corresponds with the median streak; but in C. lloydii, C. americanus and A. oligo- 

 podus, the most aboral portion has a more composite character, in as much as thick-walled neniato- 

 cysts become more common in this part. In Arachnaclis lobiancoi the craspedion region remains of 

 exactly the same structure as the median streak of the ciliated tract region; also the craspedia are 

 very short here and form a very inconsiderable section between the ciliated tract region and the 

 "acontia". 



In Arachnanthus oligopodtis the craspedia form the longest part of the filament On the dis- 

 appearance of the groove of the ciliated tracts by the coalescence of the borders of the groove the 

 ciliated tract passes on towards the aboral side as a small streak on either side of the mesentery, but 



