OF THE IIYDROMEDUSJE. 383 



the Geryonidae arc divided into two great groups: one group including those specie's in 

 which the intcrradial tentacles are retained by the adult, and the other including those 

 in which they disappear before maturity is reached. 



In our species, however, there is no invariable rule. Most adults retain all four of 

 them; but individuals with only three, two, one, or with none at all are sometimes found. 

 It is possible, and in fact probable, that this is true of other species also, and that the 

 presence or absence of these tentacles cannot be used as a diagnostic characteristic. 



The third set of tentacles, the secondary radials, are always present in the adult. 

 They appear as small buds, fig. 9, g, in the young medusa, and grow throughout life. 

 They arc very elastic and may be stretched out to four or five times the diameter of the 

 bell, and they are seldom contracted to less than twice this diameter. They arc hollow 

 and their lasso-cells are arranged in prominent rings along the whole length of the ten- 

 tacle. 



Summary of the Development of LiuroPE. 



The following features in the life-history of Liriope are especially important as a basis 

 for comparison with other hydromedusa? in the attempt to trace the origin of alterna- 

 tion. 



1. Each egg gives rise to no more than one adult medusa, and there is no alternation 

 of generations or asexual process of multiplication. 



2. The segmentation cavity persists as the digestive cavity, and the embryo is not a 

 solid mass of cells at any stage of its development. 



3. The process of delamination which results in the formation of the two germ lay- 

 ers takes place rapidly over the whole of the spherical blastoderm. 



4. The metamorphosis is gradual and is not divided into well-marked stages separated 

 from each other by sudden changes; but it maybe divided into a planula period, a 

 hydra period and a medusa period, although certain characteristics of the medusa appear 

 during the planula and hydra periods, and certain characteristics of the hydra are re- 

 tained after the medusa period is reached. 



5. During the planula period the spherical embryo consists of a ciliated ectoderm, 

 and a capacious digestive cavity which has no opening to the exterior and is bounded 

 by a single spherical layer of endoderm cells concentric with the ectoderm, but sepa- 

 rated from it by the gelatinous umbrella which is at first spherical and of uniform thick- 

 ness. 



6. The planula is converted into a hydra by the union of the ectoderm and endo- 

 derm at the oral pole, where the two layers become perforated to form the mouth, around 

 which the ectoderm cells become differentiated into a sharply defined oral area or peri- 

 stome, on the periphery of which four solid hydra-tentacles are developed. The hydra 

 is free, does not multiply asexually and has a gelatinous umbrella. If this were absent 

 it would be very similar to the actinula of Tubularia. 



7. As the hydra becomes converted into the medusa the peristome becomes pushed 

 inwards to form the sub-umbrella, at the top of which the mouth is situated; while the 

 digestive cavity becomes converted into a dome with its edge at the bell margin. The 

 ex-umbral and sub-umbral layers of endoderm are thus brought close together and they 



