3(j THE VOYAGE OF H.M.S. CHALLENGER. 



The trunk, from which these nullifications proceed, being divided into two main 

 branches, two systems of supporting lamellae are present, radiating respectively one from 

 each of these ; the two systems are contiguous in the median plane of the muscle, 

 thus giving rise to repeated fusion of their respective supporting plates. The muscular 

 layers become consequently detached into bundles of muscles, and the endodermal 

 muscles partially transformed into mesodermal. The pleatings of the same system 

 rarely become connected with one another by lateral lamellae, though this is more 

 frequent at the point where the circular muscle passes transversely through a septum ; 

 indeed here they are often connected to such an extent that a great part of the muscular 

 fibrillar runs for some time entirely in the mesoderm. 



In the upper half of the wall we find small endodermal evaginations, which grow like 

 glands into the underlying connective substance (PI. VIII. fig. 4), and show a streak of 

 blackish colouring in transverse section. Their csecal end nearly reaches the ectodermal 

 epithelium, but is always separated from it by a thin partition of connective substance, 

 so that we never find small openings comparable to cinclides. The colouring is caused 

 by the accumulation of black pigment granules in the endodermal epithelium. The 

 endodermal muscular layer is not so thickly pleated throughout the region of evagination 

 as in other parts of the wall. The evaginations seemed to me to be present only in the 

 intraseptal spaces, but they were so frequent there that many intraseptal spaces showed 

 three of them in radial section. 



The oral disk and tentacles did not admit of detailed examination ; enough that 

 both parts possess an ectodermal, richly-pleated muscular lamella. The septa, on the 

 contrary, are of special interest, firstly, from the constitution of the muscular system, 

 and, secondly, from their arrangement (PI. VIII. figs. 3 and 5). 



The longitudinal muscles of the septa are developed to an extent which I have 

 never met with in any other Actinia ; they form thick swellings, showing an ex- 

 tremely delicate figure in transverse section. The pleatings of the supporting substance, 

 which are covered with muscular fibres, are thickly branched, lie closely together, and 

 pass one between the other in such a way as to form in transverse section what one 

 might almost call a " meandrous complication," although the supporting layers never 

 absolutely become fused. The mass of the muscle actually projects above the surface of 

 the septum, and presents a mushroom-shaped appearance, caused by the constriction at 

 its base. 



The muscular swellings lie on the septa till within a short distance from the wall and 

 from the oesophagus ; there the muscular fibrilke extend in a smooth layer, and only 

 become again more closely pleated when still nearer the wall. A slight parietobasilar 

 muscle on the side of the transverse muscular layer corresponds to this second longi- 

 tudinal cord. 



All the septa are grouped in pairs in such a way that, with the exception of the two 



