%8 ZOOLUGY. 
mouth, is found in their midst in such a way that its two 
ends have a tentacle opposite each, and the other four dis- 
posed two on one side and two on the other. Within, the 
organs arise at points corresponding to the position of those 
outside. The semi-partitions, twelve in number, begin as 
mere ridges, which extend in pairs from the anterior end of. 
the stomach along the oral wall toward its border.’? Adult 
Actiniz sometimes, though rarely, subdivide longitudinally, 
but it is not uncommonly observed in the corals, in which 
cases only the heads and stomachs divide, the general cav- 
ity remaining common to the two. 
The development of Actinia mesemdryanthemum has been 
traced by Lacaze-Duthiers. The young Actinia attains 
maturity without any metamorphosis. The egg is supposed 
to undergo segmentation within the ovary. In the state in 
which the embryo was observed by Lacaze-Duthiers it was 
oval and surrounded by a dense coat of transparent conical 
spinules. Soon the two primitive germi- 
nal layers (ectoderm and endoderm) 
were observed. Two lobes next appear 
within the body ; these subdivide into 
four, eight, and finally twelve primitive 
lobes. This stage is represented by the 
corresponding stage of the coral (Fig. 55, 
B). Not until after the twelve primitive 
lobes are fully formed do the tentacles 
Fig. 51.-—-Ciliated larva begin to make their appearance. When 
eae alee 3Pias. the first twelve tentacles have grown out, 
pore) Beans twenty-four more arise, and so on, until 
“After Meteehnikod. with its increasing size the Actinia is 
provided with the full number peculiar to each species. 
Lacaze-Duthiers observed the same changes in two species 
of Sagartia, and in Bunodes gemmacea. Fig. 51 represents 
the ciliated gastrula of an unknown polyp allied to Kalliphobe. 
While Metridium and Bunodes are types of the ordinary 
form of Actinoids, certain forms, like Haleampa producta 
Stimpson (Fig. 52), are quite long and live fixed in the 
mud or sand. Allied to Halcampa is Edwardsia, which 
