THE ANATOMY OF ACTINIA MESEMBRYANTHEMUM. 215 



mesembryanthemum, Anthea cereus, Tealia crassicornis, Sagartia 

 parasitica, and Actinoloba dianthus, with a slight variation in the 

 case of T. crassicornis (see note post), it would be wrong for some 

 of them if we added the eggs and the organs, for the simple reason 

 that these are carried in different situations in different species. 



To explain the diagram it is hardly necessary to remind the reader 

 that as a rule and with a slight exception* one sextant in an Actinia 

 answers for all, each Actinia being but an aggregate of similar com- 

 partments, usually six in number — T. crassicornis having a pre- 

 ference for ten, while a few run to four and eight. Fig. 2, pi. XI, 

 exhibits the grouping of the compartments in sextants, and is an 

 exact picture of the base of A. mesembryanthemum. 



Now to construct a diagrammatic figure we must take one of the 

 sextants of the base, a 1 to a 2 , and build up (see Figure) upon it a 

 complete compartment, with disc and stomach-wall to close it in, and 

 then, by suitable incisions, divide the disc and stomach-wall each into 

 two parts, draw the parts aside, and sever them where necessary from 

 the septa that carry them. We shall thus disclose to view seven 

 pairs of intermediate septa, but in order to allow of these being drawn 

 in perspective, we must go a step further and separate the large pair 

 of primary septa a 1 , a 2 , which form the boundaries of the sextant, 

 and by turning these septa, as if on a hinge, away from the parts 

 which we seek to exhibit, and allowing them to take with them the 

 pieces of the disc and stomach- wall which we have excised, we produce 

 the result seen in the diagram, Fig. 1. It must be borne in mind 

 that the disc rests upon and is attached to all the septa, while the 

 stomach wall is attached, not to the septa called quaternaries, but 

 only to primaries, secondaries, and tertiaries ; these carry the 

 stomach-wall, and by their powerful contractions and- expansions 

 give it its elasticity and force as a compressorium.\ 



It is no doubt true that every now and then abnormal cases occur, 

 particularly in A. dianthus, in which a few abortive and extra septa 

 that do not reach the disc at all, are found ; but I think they are 

 excptions, and the diagram may be relied on as the normal form. 



* This exception refers to the twogonidia carrying compartments, and of 

 these there is one only in A, dianthus. See Gosse (I.e.) 



f I think it doubtful whether the quaternaries in T. crassicornis ought not 

 in fact to be treated as actually reaching the back of the stomach. wall, but 

 if they do reach it, it is by an arch so elevated that it thins off into a very 

 slight rib-like projection running along the under side of the disc and the 

 back of the stomach-wall. See Part II. 



