48 ON THE ORIGIN AND [CHAP 



cesses on the undersides of the segments to which 

 they belong. 



As already mentioned, it is possible that at one time 

 the vermiform state of the Homomorphous insects 

 which, as we have seen, is now so short, and passed 

 through at so early a stage of development was 

 more important, more prolonged, and accompanied by 

 a more complete condition of the internal organs. 

 The compression, and even disappearance of those em- 

 bryonal stages which are no longer adapted to the 

 mode of life which do not benefit the animal is 

 a phenomenon not without a parallel in other parts 

 of the animal or even of the vegetable kingdom. 

 Just as in language long compound words have 

 a tendency to concision, and single letters sometimes 

 linger on, indicating the history of a word, like 

 the "I" in " alms," or the "b" in " debt," long 

 after they have ceased to influence the sound ; so in 

 embryology useless stages, interesting as illustra- 

 tions of past history, but without direct advantage 

 under present conditions, are rapidly passed through, 

 and even, as it would appear, in some cases alto- 

 gether omitted. 



For instance, among the Hydroida, in the great 

 majority of cases, the egg produces a body more 

 or less resembling the common Hydra of our ponds, 

 and known technically as the " trophosome," which 

 develops into the well-known Medusae or jelly-fishes. 

 The group, however, for which Prof. Allman has pro- 

 posed the term Monopsea, 1 and of which the genus 



1 Monog. of the Gymnoblastic or Tubularian Hydroids. See also 

 Hincks, British Hydroid Zoophytes. PL x. 



