20 PROFESSOR A. M. MARSHALL. 



gonophores of the Hydromedusee, until finally the extreme condition 

 seen in Hydra is produced. 



We do not know the causes that determine the period, whether late 

 or early, at which the reproductive organs ripen, but the question is 

 one of great interest and importance and deserves careful attention. 

 The suggestion has been made that entire groups of animals, such as 

 the Mesozoa, are merely larvae, arrested through such precocious 

 acquiring of reproductive power, and it is conceivable that this may be 

 the case. Mesozoa are a puzzling group in which the life history, 

 though known with tolerable completeness, has as yet given us no 

 reliable clue concerning their affinities to other animals, a tantalising 

 distinction that is shared with them by Rotifers and Polyzoa. 



Distortion of a curious kind it seen in cases of abrupt metamorphosis, 

 Avhere, as in the case of many Echinoderms, of Phoronis, and of the 

 metabolic insects, the larva and the adult differ greatly in form, habits, 

 mode of life, and very usually in the nature of their food and the mode 

 of obtaining it ; and the transition from one stage to the other is not a 

 gradual but an abrupt one, at any rate so far as external characters 

 are concerned. 



Sudden changes of this kind, as from the free swimming Pluteus to 

 the creeping Echinus, or from the sluggish leaf-eating caterpillar to the 

 dainty butterfly, cannot possibly be recapitulatory, for even if small 

 jumps are permissible in nature, there is no room for bounds forward 

 of this magnitude. Cases of abrupt metamorphosis may always be 

 viewed as due to secondary modifications, and rarely, if ever, have any 

 significance beyond the particular group of animals concerned. For 

 example, a Pluteus larva may be recognised as belonging to the group 

 of Echinoidea before the adult urchin has commenced to be formed 

 within it, and the Lepidopteran caterpillar is already an unmistakable 

 insect. Hence, for the explanation of the metamorphoses in these 

 cases it is useless to look outside the groups of Echinoidea and Insecta 

 respectively. 



Abrupt metamorphosis is always associated with great change in 

 external form and appearance, and in mode of life, and very usually in 

 nutrition. A gradual transition in such cases is inadmissible, because 

 in the intermediate stages the animal would be adapted to neither the 

 larva nor the adult condition ; a gradual conversion of the biting 



