THE ANATOMY OF THE MADREPORARIA. 5 



bet-ween the spikes (echinulations) of the costse (Figs. 6, 7) ; and 

 further, by radial canals (Figs. 3, 4, c 3 ) they open into the internal 

 longitudinal canals, which I believe, as above stated, to have, at an 

 earlier period in the history of the branch, occupied a position 

 similarly external to the corallum. The whole system which thus 

 perforates the corallum, and allows free current of fluid to even the 

 most remote parts of the colonjr, is lined by endoderm and mesoderm 

 throughout, and opens into similarly lined polyp cavities. 



The general structure of the colony is, therefore, (1) an external 

 body wall, under which and between the costse lies (2) a series 

 of external longitudinal canals opening into each other, and also 

 through the corallum, into (3) the internal canals, mainly longitudinal, 

 with radial and transverse connections, communicating in their turn 

 with (4) the ccelentera of the polyps. Into the last the external 

 longitudinal canals also open directly, through the theca. The 

 whole system is of course merely a complication of tbe primitive 

 ccelenteron. 



Of the polyps there are at least two distinct types, which are full 

 of interest as constituting the first record of marked dimorphism 

 among the Madreporaria. Both are Actinian in structure. 



Type A has in the highest sections twelve perfectly normal mesen- 

 teries, and a stomatodseum which is a simple invagination of the 

 external body wall. A little way down in the polyp, six of the 

 mesenteries, in every case the same six, assume a curious modifica- 

 tion of structure, which will be described first as seen in a series of 

 transverse sections. Fig. 8 represents the characteristic features of 

 a polyp of this type ; the mesenteries numbered 2, 4, 6, 7, 9, 11 are 

 those which undergo modification, and are diagrams of a series of 

 drawings made from the same mesentery with camera lucida at 

 different heights. 



There appears first (Fig. 8 2) an involution of the stomatodseum 

 directed towards the mesentery, on the floor of which the ectodermic 

 cells are long, but shorter at the sides. By fusion of the mesoderm 

 and obliteration of the ectoderm on each side of this involution, a 

 small canal with a definito lumen is found to be pinched off, and to 

 lie enclosed in the mesoderm lamella of the mesentery (Fig. 8 1). 

 In the neighbourhood of this involution, the endodcrmic cells 

 lining the mesenterial chamber become enormously lengthened 



