The atrophy and total disappearance of ancestrally well-marked somites fre FIG.
Lankester (2) was the first to suggest that (as is actually the fact in the Nauplius larva of the Crustacea) the prae-oral somites or prosthomeres and their appendages were ancestrally postoral, but have become prae-oral " by adaptational shifting of the oral aperture."
It is commonly assumed that the Trachylinae are forms which have lost the alternation of generations possessed by them ancestrally, through secondary simplification of the life-cycle.
If it is assumed that all these genera bore gonophores ancestrally, then medusa of similar type must have been evolved quite inde pendently in a great number of cases.
