128 SYNOPTIC COLLECTION. 



early stages of development, but afterward develop a 

 radial symmetry. 1 



Actiniae. It is now pretty well established that the 

 Actiniae are bisexual. The egg of our common sea 

 anemone, Metridium marginatum Ag. (PI. 197, figs, i-io), 

 leaves the parent form unfertilized. This is not the case 

 with all Hexactiniae, the embryo of Rhodactis being so far 

 developed when it passes into the water as to possess from 

 two to four mesenteries, while that of Aulactinia possesses 

 eight or twelve. Fig. i is an immature ovum taken from 

 the ovary. The nucleus with its nucleolus and the process 

 extending outward at one pole are clearly seen. Figs. 2-6 

 represent the segmentation of the egg ; fig. 7 is the young 

 blastula ; fig. 8, an optical section of a free-swimming 

 blastula. Some of the embryos at this stage are hollow 

 and seem to be empty, while others (fig. 9) are filled with 

 a liquid containing scattered cells. Fig. TO illustrates the 

 formation of the endoderm by delamination of the inner 

 ends of the ectoderm cells. After this stage ihe mouth 

 breaks through, making an opening to the internal cavity. 

 The young Actinian (PI. 198, figs. 1-3 ; the vertical line 

 represents the height of the anemone when expanded) 

 is a single, fleshy animal. The mouth is surrounded by a 

 limited number of hollow tentacles. It leads into the in- 

 ternal bag which communicates with the body cavity. 

 The latter is divided first by four, then by eight mesen- 

 teries which arise bilaterally. There is no indication of a 

 skeleton in the young sea anemone, neither is there so 

 great differentiation in the histological structure of the 

 animal as in more specialized Anthozoa. For these 

 reasons, and because the mesenteries arise on either side 

 of the median plane of the body, producing bilateral 

 symmetry, we regard the Actinia as a primitive rather 

 than a reduced form. When the animal grows older the 

 bilateral arrangement of parts gives way to a radial 



1 Moseley, Quart. Journ. Micr. Sci., XXII, 1882, p. 395. 



