226 



CCELENTERATA 



the oesophagus, there may be others which do not reach the oesophagus 

 and belonging to secondary, tertiary or other series (fig. 194). 



The septa support a number of important organs: the mesenterial 

 filaments, gonads, and muscles. The mesenterial filaments are thick 

 strands of epithelium, rich in glands and nettle cells, fastened like a hem 

 on the edge of the septa. Since they are much longer than the peristomial- 



pedal length of the septa, they cause 

 these latter to wrinkle and fold, thus 

 strikingly resembling the mesenteries 

 of the mammals. They envelope the 

 food and press it in, thus aiding the 

 succeeding intracellular digestion. 

 Lower down, in some species, the 

 filaments become free and form long 

 threads, acontia, rich in nettle cells 

 which are protruded for defence, 

 either through the mouth or pores 

 (cinclides) in the column. The gonads 

 only exceptionally hermaphroditic 

 lie beside the mesenterial threads 

 as thickenings of the septum (fig. 

 192, g). The germ cells arise from 

 the entoderm, but early migrate into 

 the mesoglcea of the septum (193, 0). 

 The eggs, when ripe, escape into the 

 gastrovascular cavity. The young 

 leave the parent at various stages of 

 development, sometimes as planulae 

 (fig. 197, A), sometimes as young with 

 tentacles. 



The muscles are very important, 

 morphologically. Muscles and nerves 



occur in both ectoderm and entoderm; but while the nerves are best 

 developed in the ectoderm, especially around the mouth, and extend 

 into the mesgolcea, the muscles of the ectoderm are weakly developed 

 and are mostly confined to the peristome and the tentacles. The ento- 

 dermal musculature is much stronger. Just outside of the tentacles is 

 usually a strong circular (sphincter) muscle (m) which can close in the 

 top of the column over the peristome. The septa also bear muscles, 

 transverse on one side, longitudinal on the other, the latter producing 

 ridges on the septa (fig. 193). 



FIG. 193. Section of septum of 

 Edwardsia tuberculata. ek, ectoderm; 

 en, entoderm; me, supporting layer; 

 mf, septal muscle; o, ovary; v, mesen- 

 terial filament. 



