135 



fibres are set on plates of the structureless lamella, but the 

 transverse, which are very slightly developed, have no such 

 folds. The origin and course of the longitudinal muscles 

 may be seen in Fig. II. The separate fibres never cross one 

 another, but below the filaments muscle-plates are found on 

 both sides of the mesentery, some of the most deeply attached 

 muscles crossing the free edge of the mesentery to its opposite 

 side. The fibres end at the attachment of the mesenteries 

 more or less in clumps, which seem to be connected with a 

 similar mode of attachment of the mesentery to the corallum 

 (see Fig. II,). 



The transverse muscular fibres, lying on the opposite faces 

 of the mesenteries to the longitudinal, do not appear to me to 

 extend more than half-way down the stomodoeum. They 

 run outwards mainly to the body-wall external to the 

 tentacles, and have no connection with the attachment of the 

 mesenteries to the corallum. The tentacles being entocoelic 

 cannot be connected with these muscles in any way.* The 

 longitudinal muscles alone contract the polyp, the expansion 

 being due to the elasticity of the polyp following the relaxa- 

 tion of the same muscles. The transverse muscles would seem 

 to be present solely for the purpose of opening the stomo- 

 doeum for the reception of food, though they might, by 

 drawing together the external body-wall and stomodoeum, 

 assist slightly in pushing out the tentacles. 



Mesenteries. — The first cycle of mesenteries— 48 in 

 number, all reaching the stomodoeum — should be the pairs on 

 either sides of primary, secondary and tertiary septa. There 

 are then typically 48 further mesenteries of a second order, 24 

 pairs on either side of quaternary septa. These do not reach 

 the stomodoeum, but start from the peristome near the mouth, 

 only very exceptionally being attached to the stomodoeum 

 for the whole or part of its length. I have not cut serial 

 sections of any polyp with a third cycle of mesenteries, but 

 from dissections it is clear that the latter are attached almost 

 in the same position as the secondary mesenteries. 



The mesenteries vary considerably in size, but their 

 general appearance may be seen in figures I. and II. In any 

 polyp the secondary mesenteries are usually nearly of the 

 same size, but the primary may vary somewhat, in the smaller 

 polyps 24 being sometimes larger and extending deeper 

 into the calicle marking out the original primary and 

 secondary septa. 



The filaments of the primary mesenteries extend down 

 from the thickenings of the stomodoeum, and form an irregulai 



• The tentacles over the primary septa at each end of the calice, hence between 

 the two pairs of directive mesenteries, are never contracted to the same extent as the 

 literal tentacles, being drawn in principally by the general contraction of the polyp. 



