-8 CERIANTHARIA. 



and indeed we have no other factor of service, if we try to find the homology of the varying arrange- 

 ments of mesenteries in Anthozoa. 



On the other liand, for the contention that the siphonoglyph and the directive mesenteries in 

 Ceriantharia are ventral, the only argument to be adduced is, as we have urged, the ]i\'potlietical 

 agreement in the order of succession of the first (6) 8 mesenteries, but after the proof of the irregularity 

 in the development of these mesenteries, such alleged agreement cannot weigh heavily in balancing 

 the arguments for the ventral or dorsal character of the directive mesenteries and siphonoglyph in 

 Ceriantharia. On the supposition that the directive mesenteries in Ceriantharia are ventral, the mesen- 

 terial musculature of this group in the 6 (8) mesentery stage, will, as I have urged above, face exactly 

 the opposite way to that seen in the other Anthozoa, and further new formations of mesenteries after 

 the protomesentery stage would take place in the direction of the dorsal instead of the ventral side 

 as is the case in Zoantharia and, in a way, in Actiniaria and Madreporaria also — a state of things 

 which is quite inexplicable. 



On the otlier hand, by making use of my terminology and regarding the directive mesenteries 

 of Ceriantharia as dorsal, we have no difficulty in eliminating the diversity thus occasioned in the 

 order of development of the first mesenteries, for a merely trifling divergence in the time of formation 

 of the directive mesenteries, by which they appear as second in the series of mesenterial development 

 instead of third, brings the formation of the first 6 mesenteries in Ceriantharia into harmony with 

 the series of development which is assumed at least on phylogenetic grounds for the first six mesen- 

 teries in Actiniaria. We need not compare more than the first 6 mesenteries in Ceriantharia with the 

 like number of mesenteries in Actiniaria, because the protomesentery stage in the former group is 

 completed with the appearance of the first 6 mesenteries. 



I range myself then, as I declared before, on the side of van Beneden, when he maintains 

 in opposition to Boveri and Mc. Murrich, that Ceriantharia do not pass through an Edwardsia 

 stage. P'or tlie first, it must be noted that during the period of development Ceriantharia have no 

 definitely marked off 8-mesentery larval stage, whilst the larval forms of Actiniaria, Madreporaria, and 

 in all probability Zoantharia also, remain in that stage a good while. For the second, there appears 

 after the 6-mesentery stage a zone of new mesentery formation, on the directive plane produced, 

 directly opposite the directive mesenteries. Tliis enables us to distinguish two periods in the 

 development of the mesenteries in Ceriantharia, the first period, during which the first 6 mesenteries 

 make their appearance — first a couple vertical to the directive plane, then a second couple at the 

 corner of the mouth oi)posite the directive chamber which is later to appear and lastly, immediately 

 after the forming of tlie second couple, the directive mesenteries, at the other corner of the mouth — 

 and a second period, during which a new formation of mesenteries takes place in a multiplication 

 zone in the chamber bounded by the second couple. The demarcation between the protomesenterial 

 and the mesenterial stage in Ceriantharia thus occurs after the formation of the first six mesenteries, 

 from which point of time a distinct zone of mesenterial formation presents itself, and not after the 

 8-mesentery stage, which is not a distinctly marked off stage at all of the development in this group. 

 No doubt it may be objected, that neither does the 6-mesentery stage itself, as far as we know at 

 present, mean a pause in the development of the Ceriantharia, seeing that so far the development 



