METAZOA COELENTERA. 117 



scarcely anything is known of the development of these 

 primitive forms, since this knowledge would doubtless 

 throw light on the phylogeny of the Anthozoa. 



Monoxenia (PL 161, greatly enlarged) never secretes 

 a skeleton or any hard parts. Its tubular fleshy body has 

 only eight tentacles (PI. 161) and these are hollow pro- 

 longations of the internal cavity (PI. 162, fig. i, longi- 

 tudinal section of the body) . The internal structure is 

 simple, since there are only eight mesenteries (PI. 162, 

 fig. 2). These bear clusters of eggs which are also seen 

 in fig. i. The cross section is made near the central part 

 of the body cavity which is marked by the dotted line. 

 It cuts through a mesentery on one side and between two 

 mesenteries on the other. 



Hartea is another simple form with eight tentacles and 

 mesenteries, but in this case the base of the body and 

 the tentacles are provided with star-shaped spicules. 

 Haimea has variously shaped spicules and this genus also 

 possesses thread cells or nematocysts. * 



In these three genera no ciliated groove (siphono- 

 glyph) at one end of the mouth opening has been 

 described, and probably none exists. The last two gen- 

 era have a skeleton consisting of spicules. They are 

 found only on the base or in the body walls and are 

 never secreted in the body cavity ; there are therefore no 

 false nor true septa. This can be said not only of these 

 genera but also of all living Alcyonaria, and it is prob- 

 able that this changed and reduced condition of the 

 skeleton has been brought about since the tabulate 

 ancestors of the Alcyonaria flourished in Palaeozoic 

 times. 



From single forms we pass to simple colonies. Cornu- 

 laria cornucopiae Lam., is a simple colony without spic- 

 ules but with more or less horny matter. It is interesting 

 to note that in this species the ectoderm secretes a horny 

 sheath (PI. 163, fig. i) which reminds one of the external 

 skeleton of the hydroids. 



