80 SUMMARY OF CURRENT RESEARCHES RELATING TO 



rays, perhaps an abnormal change arising in the later larval life. With 

 these changes on the actinal side there must have been compensating 

 abactinal changes. One line evolved through the elongation of rays, 

 flexibility, and the increase in size of the oral region by the (a) method, 

 from the primitive Hudsonasterida into the derived Palfeasteridffi, 

 Promopaleeasteridffi, Xenasteridae, and Neopalfeasteridte. This was the 

 most satisfactory line of evolution, resulting in the greatest variety of 

 genera. Another line, starting with Hudsonaster as radicle, was estab- 

 lished through the increase of the body cavity by the {b) method, giving 

 rise to the Palasterinidas. This was not so prolific in genera as the 

 former. Finally, a third phyletic line was estabhshed through larval 

 adaptation, causing a sixth ray to develop and thereafter pairs of rays, 

 resulting in the multi-rayed Asterids of the family Lepidasteridre. 



Coelentera. 



Inheritance in Asexual Reproduction of Hydra.* — K. S. Lashley 

 finds that within a wild population of Hijdra viridis there are heredi- 

 tarily diverse races w^hich differ in the number of tentacles at separation 

 from the parent, in their size at a given age, and less certainly in other 

 characters. The differences between such races are permanent so long 

 as the races are kept under the same environment. The evidence favours 

 the view that the differences are truly genotypic (i.e. due to hereditary 

 constitution), but the reservation is made that they are possibly the result 

 of differences in the age of the clones. (A clone is a family descended 

 from a single individual by asexual reproduction.) 



In the absence of selection the strains remain distinct. Within popu- 

 lations there is a correlation between the characteristics of parents and 

 progeny and of other close relatives, which is largely due to the existence 

 of these diverse strains. Within the clone there is no significant cor- 

 relation between the variations of close relatives in the initial number of 

 tentacles. Within, the clone there is a slight correlation between the 

 number of tentacles of the buds and the number of tentacles which theii* 

 parents bear when each bud is produced. 



Diversities of environment tend to produce like-variations in parents 

 and offspring, and this likeness tends to disappear when the environ- 

 mental cause is removed. The existence of such environmental agents 

 is sufficient to account for the ancestral correlations found, even though 

 there is no inheritance of variations. 



Continued selection of variants in tentacle-number results in changes 

 in the vigour of the selected groups. This results in an apparent 

 diversity of the differentially-selected groups, but the diversity persists 

 only during selection, and disappears at once when selection is dis- 

 continued. Variation in the number of tentacles of Hydra viridis is 

 not inherited. 



There is a positive correlation between the variations in the size of 

 parent and offspring within the clone. The statistical evidence of an 

 external cause of this correlation is presented, but from general con- 



* Journ. Exper. Zool., xix. (1915) pp. 157-210 (10 figs.). 



