6o CERIANTHARIA. 



discover, no investigator has brought out the difference between the structure of the cnido-glandular 

 tract in the cihated tract region and in the single portion below tlie ciliated tracts. In order to 

 recognise the homologies between the different filament portions of the sterile and fertile mesenteries 

 in the various Ceriantharia, it is necessary however to distinguish as I have already done above, 

 these cnido-glandular tracts from each other. The middle tract in the ciliated tract region I have also 

 called above the median streak or perhaps better the spirocyst-glandular tract'), characterised 

 by the presence of numerous thin-walled nematocysts and very numerous mucus cells. For the wavy 

 filament on the sterile mesenteries of the third and fourth cycles and also for its analogue on the fertile 

 mesenteries, I have retained however the old designation cnido-glandular tract. It is characterised 

 by the presence of numerous, large and broad, thick-walled nematocysts, and of closely packed granular 

 gland cells, whilst spirocysts are either absent or very rare. *The first tract, as A. von Heider long 

 ago (1879) and later van Beneden (1S98) pointed out, forms a direct continuation of the ectoderm 

 of the stomatodaeun: and in certain species such as Ccriaiif/nis lloydii, Botriicnidifcr iiorvegicus is 

 prolonged on certain mesenteries below the cnido-glandular tract as a craspedion region. The cnido- 

 glandular tract on the other hand and van Ben ed en's "acontia" and botrucnidae are to be regarded 

 as special differentiations of the filament. 



The ciliated tract region of the filament is composed of the following parts: 

 i) a median streak or spirocyst-glandular tract, which is a direct continuation of the 



ridges of the stomatodaeum. 

 2) two ciliated tracts one on either side of the median streak, which are a direct continuation 



of the ectoderm and mesogloea in the furrows of the stomatodaeum. 



The ectodermal origin of the filament is adopted by v. Heider (1S79) and Boveri (1889) but 

 denied by O. and R. Hertwig (1879). Later van Beneden (1898^ gave several grounds for an 

 ectodermal origin of the filaments, and shewed the connection between the filaments and the stoma- 

 todaeum. I subscribe unreservedly to the oijinions expressed by van Beneden on the genetic origin 

 of the ciliated tract region. Of its relation to the cnido-glandular tract, the "acontia" and botrucnidae, 

 I shall say more further on. 



As van Beneden has described, the attachment of the mesenteries is shifted in the most aboral 

 part of the stomatodaeum. Instead of the junction of the mesenteries taking place in the longitudinal 

 furrows, as it does over the greatest part of the stomatodaeum, the line of mesenterial attachment 

 gradually creeps up to the ridges, so that where the stomatodaeum passes over into the filaments, the 

 longitudinal furrows come to lie between the attachments of the mesenteries. 



Let us first consider a median streak. The ridge, more or less tongue-like in transverse section, 

 in the most aboral parts of the stomatodaeum has become considerably shortened in all sj)ecies with 

 the exception of Botrucnidifer norvcgims, so that the ridges soon get broader than they are high. 

 Both the ectoderm and the mesogloea partake in this flattening so that the latter forms a process 

 more or less T-shaped in transverse section. The nerve layer and the longitudinal nmscle layer at 



') When speaking of nematocysts in the filaments, writers hardly ever make any distinction between thick-walled 

 and thin-wallcd nematocysts. The latter I call briefly spirocysts, without taking any decided attitude on tlie question 

 whether the two forms do or do not shew any difference in their intimate structure or function (compare Will 1909, 

 Moroff 1909). 



