SEDENTARY HABITS 37 



extinct echinoderms are accordingly divided into 

 two grrades : A. Pelmatozoa ; B. Eleutherozoa 



(s. sir.). 



The remarkable prevalence of the sedentary 

 habit amongst the lower (i.e. invertebrate) animals 

 seems to indicate that something peculiarly prim- 

 ordial lies at the back of the phenomenon, and 

 makes it desirable that it should be expressed in 

 physiological classification. I venture to assume 

 the liberty to employ two contrasting terms in a 

 wide physiological sense, which have been used 

 already in a narrow morphological sense, namely, 

 Eleutherozoa or free animals, and Statozoa or 

 sedentary animals. 1 



The effects of the sedentary habit upon 

 the growth and organisation of the animals 

 accustomed to it have been amply discussed by 

 Semper, Lang, and others, and need not be 

 described here ; but it would be instructive if 

 we could establish the existence of a primary 

 function which formed the physiological basis 

 of all this varied fixation occurring in widely 

 separated morphological groups. There is an 

 extra value in this inasmuch as we might by 

 its means obtain some light upon a number of 

 perplexing problems to which no direct answer 

 is possible, e.g., as to whether the fixed hydroid- 

 phase is more primitive than the free medusoid- 

 phase in Hydromedusae ; or again as to whether 



1 In morphological nomenclature Statozoa is synonymous with 

 Pelmatozoa {vide Bather, op. cit.) 



C 2 



