ACTINOZOA. 225 



are met with. In rare cases all the individuals are hermaphrodite, 

 e.g., Gerianthus. 



The embryos produced from the fertilised ovum, which undergo 

 a complete segmentation, are frequently born alive as ciliated larvae, 

 and possess an internal gastric cavity, and an oral aperture situated 

 at the pole, which is directed backwards during movement. They 

 then fix themselves by the pole opposed to the oral aperture and 

 protrude in the region of the mouth first two, then four, eight, 

 twelve, etc., tentacles ; in the Octactinia eight tentacles at once. 



In the Polyactinia, the tentacles and mesenteric pouches of which 

 are arranged in multiples of six, it was till recently erroneously believed 

 with M. Edwards that six primary mesenteries were first developed, 

 then six secondary between them ; then twelve were formed, then 

 twenty-four, etc., so that mesenteries of equal size were of equal age 

 and belonged to a cycle formed at one time. Lacaze Duthiers 

 however produced proofs that the increase of mesenteries and of 

 tentacles follows an entirely different law of growth, and that these 

 structures in the first phases of development show a bilateral 

 symmetry; and it is only later that the six radial symmetry 

 appears by the equalization of the alternating elements of unequal 

 age. A remnant of the primitive bilateral symmetry is moreover 

 often preserved in the elongated mouth slit, which falls in the 

 plane of the two primary tentacles. 



Amongst the Polyactinia the very young larvae of the Actinia 

 (A. meseinbryantheuium, Sagartia, Bunodes) have been most accu- 

 rately investigated. They are small ciliated planulse, one pole of 

 which is somewhat drawn out and bears a tuft of longer cilia. The 

 opposite end of the body is flattened and pierced with a mouth. 

 This leads by a short oesophageal tube, which arises by invagination, 

 into the narrow gastric cavity. The first differentiation consists in 

 the appearance of two folds placed opposite each other, which divide the 

 gastric cavity into two unequal chambers. The mouth is drawn out 

 in the form of a longitudinal slit symmetrical with and at light 

 angles to these primary mesenteric folds ; so that by means of 

 them the position of the median plane can be determined. Two new 

 folds soon arise in the larger chamber, which we will call the anterior ; 

 these lie opposite to one another and symmetrically with the median 

 plane ; so that four chambers are now present, an anterior, a 

 posterior, and two smaller lateral ones. A third pair of folds are 

 then developed in the posterior space, and a fourth pair follow 

 quickly in the lateral chambers : the fourth pair are slightly smaller 



\l 



