^ COELENTEBATA 83 



are secreted by the overlying endoderm cells, these increase in 

 number and develop basal longitudinal muscular tails. The muscles 

 hrst appear on the ventro-lateral mesenteries, and afterwards on the 

 others. Those on the ventral mesenteries are turned away from one 

 another, and those on all the other mesenteries are developed on that 

 side of each mesentery which faces towards the ventral mesentery of 

 its own side. It follows that the muscle thickenings on the dorsal 

 mesenteries are also turned away from one another. 



The dorsal and ventral couples which are attached to the ends of 

 the stomodaeum are known as directive mesenteries. In many 

 Actinozoa the ends of the stomodaeum are developed into strongly 

 ciliated grooves— the so-called siphonoglyphes or gonidial grooves. 

 In practically all, including all Alcyonaria, one end of the stomodaeum 

 is thus niodified ; this is called the ventral end,, and to this end the 

 ventral directive mesenteries are attached. 



The arrangement of muscle thickenings just described is the same 

 as that which persists for life in the family Edwardsiae, in which 

 family also, as in the young Urticina, there are only eight fully 

 formed mesenteries and eight tentacles. Urticina fheieiom passes 

 through an "Edwardsia" stage in development, and this has been 

 proved to be true of every Hexactinian whose development has been 

 worked out. 



In the very oldest specimens of Urticina which Appellof was 

 able to rear he found that extra mesenteries were being developed. 

 These extra mesenteries consist of two on each side, four in all, and 

 each of the new mesenteries was in such a position as to make a pair 

 with a ventro-lateral or lateral mesentery, and each bore a muscle 

 thickening facing the muscle of its fellow in the pair (see Eig. 67). 

 Such a pair, consisting of two mesenteries facing one another, is to be 

 carefully discriminated from a couple, the two mesenteries forming 

 which are situated at symmetrical points on opposite sides of the 

 stomodaeum. 



Since the dorsal and the ventral couples of mesenteries may also 

 be regarded as forming two pairs, we reach in this way a total of six 

 pairs of principal mesenteries, and this is what is known as the 

 typical Hexactinian arrangement, the most widely distributed 

 arrangement amongst Zoantharia. The powerful sphincter muscle 

 which in the adult closes the mouth was represented in the oldest of 

 the artificially-reared specimens merely by a thin sheet of circular fibres. 



Appellof has also examined the development of a species of the 

 commonest genus of sea-anemones. Actinia. In this species, however 

 (^Actinia equina), the earlier part of the development is passed through 

 in the coelenteron of the mother, and Appellof was unable to obtain 

 a complete series of stages of this form ; however, one or two points of 

 interest were made out. 



The endoderm originates by proliferation from the ectoderm cells ; 

 a proliferation probably confined to one pole. A hollow two-layered 

 planula larva is formed which is devoid of a mouth ; but a mouth is 



