24: 



CCELENTERATA. 



"secondary" and "tertiary" mesenteries according to their relative 

 length. Below the level of the bottom of the oesophagus the inner 

 edges of even the principal mesenteries are free (fig. 120, f); and 

 the intermesenteric compartments thus all open freely into a com- 

 mon space, while they may also communicate with one another by 

 means of perforations in their bounding mesenteries, placed near 

 the point where the latter join the upper end of the gullet. Lastly, 



Fig. 121. — A, Actinia mesevibiyanthcwwn, one of the Sea-anemones (after Johnston); B, 

 section of the same showing the mouth (a), the oesophagus (b), the body-cavity (c), and one of 

 the mesenteries (;«). 



attached to the faces of certain of the mesenteries, towards the 

 lower end of the body, are the band-like reproductive organs (fig. 

 120, r). 



As regards the development of the mesenteries, Lacaze-Duthiers has 

 shown that in the Actinidce and in Astroides the first step is the appear- 

 ance of a single pair of mesenteries developed at right angles to the oral 

 fissure, nearer to one side than the other, so as to divide the body-cavity 

 into two unequal chambers (fig. 122, 1, 1). In the larger of these cham- 

 bers appear the two next mesenteries (2, 2), one on each side. Two 

 additional mesenteries (3, 3) next appear in the smaller chamber, this 

 making in all six mesenteries ; but this condition is evanescent, and two 

 further septa (4, 4) are developed on the opposite side of the first-formed 

 mesenteries to the third pair. At this stage, therefore, there are eight 

 mesenteries in all (fig. 122) ; but two further pairs of mesenteries are 

 produced, raising the number to twelve, and completing the series of the 

 " principal" mesenteries. 



According, therefore, to the observations of Lacaze-Duthiers, the 

 twelve " principal " mesenteries of Astroides are developed in six pairs, 

 which are produced independently and at separate times ; and this 

 arrangement seems to hold good in the typical Sea-anemones. The 

 two pairs of " principal " mesenteries which correspond with the opposite 

 extremities of the longitudinal mouth, are known as the "directive" mes- 

 ■enteries. The twelve mesenteries which succeed the twelve " principal " 



