IV 



COELENTERATA 



85 



just as in Uriicina crassicornis. On the other hand, in the species 

 Metridium marginatum which has been examined by McMurrich 

 (1891), and Sagartia parasitica and Adamsia palliata, which have 

 been examined by Faurot (1903), although the eggs are expelled 

 from the mother previous to fertilization yet the endoderm appears 

 to be formed by proliferation from the outer cells of the embryo, that 

 is, from cells which become the ectoderm; and in the two latter species 

 examined by Faurot this occurs at a very early period of development. 

 On the whole this method of forming the endoderm appears to be 

 much commoner than the method of imagination among Zoantharia, 

 and it is the only method recorded for Alcyonaria. In many cases 

 the endoderm cells are so swollen that, when the mesenteries appear, 

 the whole coelenteron is choked up by these cells and only slit-like 

 remnants of the cavity of the coelenteron remain. 



The two families of Cereanthidae and Zoanthidae are formally 

 classed along with the Hexactiniae as Zoantharia, but they exhibit a 

 very different arrangement of mesenteries and in each case the erg 

 develops into a most character- 

 istic larva. A good summary of 

 what is known about the larvae 

 of both families is given by 

 Carlgren (1906). 



The larva of the Cereanthidae 

 is termed Arachnactis, and it is 

 characterized by the excessive 

 prolongation of the free- 

 swimming stage. Two sets of 

 tentacles, an inner and an outer, 

 and numerous mesenteries are 

 developed whilst the larva still 

 continues to swim. Of these 

 mesenteries, there is a ventral 

 pair of "directives" attached to 

 one end of the stomodaeum, and 

 two " couples " attached to its 

 sides. The space which should be 

 occupied by the dorsal directives is 

 at first empty, but young mesen- 

 teries appear in it later, varying in 

 number with the age of the larva. 

 They are formed alternately to 

 the right and to the left of the 

 median line, and at first they are short, only later reaching the 

 stomodaeal wall. 



The larva of the Zoanthidae appears under two varieties, in one of 

 which (Zoanthella) there is a transverse girdle of strong cilia, whilst 

 in the other (Zoanthina) there is a longitudinal band of cilia as 

 locomotor organ. It develops no tentacles until twelve mesenteries 



FIG. 64. The Arachnactis larva of Cereanthus 

 membranaceus. (After Carlgren.) 



