A REMARKABLE FORM OF SKELETAL ELEMENT IN THE 

 LITHISTID SPONGES 



(A Case of Analogical Resemblance) 



By H. V. Wilson 



With Four Text Figures 



Sponges, like other groups, are rich in the structural resemblances 

 which are due to common descent, resemblances which involve nu- 

 merous organs in the individual animal and wliich are of such a com- 

 plex intricate character, striking so deeply into the constitutional 

 make-up, that it is impossible to think of them as due to any natural 

 cause save kinship. But as we stud}' these infinitely variable animals, 

 we encounter resemblances which fall in another category, resem- 

 blances which involve only some special detail of structure or at most 

 a few features, which in their actual functioning are correlated in 

 physiological-mechanical function. Such resemblances are certainly, 

 in many cases at least, not inheritances from a common ancestor. "We 

 group them together as analogical but they fall in two subdivisions : 

 (1) those involving features which are called out in each individual 

 organism by the stimulus of an environment, and which do not appear 

 when that environment is changed; and (2) those involving features 

 which are racial characteristics, viz., characteristics that have arisen 

 and become fixed, in some way, during the course of evolution and 

 which continue to appear under different sets of environmental con- 

 ditions. 



Analogical resemblances of this latter class are due to the inde- 

 pendent occurrence of the same variation (or cumulative series of 

 variations), in different idioplasms. How such germinal changes, 

 mutations as we often call them, are brought about physiologically, is 

 a question that is being actively asked by many students of heredity, 

 especially by the experimental evolutionists, of today. Along with 

 the directly experimental attacks, descriptive work has its use in 

 locating facts which at some time it may be worth while to put under 

 the fire of experiment. 



Partly, at least, in pursuance of this idea I wish to record a case 

 of resemblance which is certainly analogical, and which, it is practi- 

 cally certain, is racial. It involves the shape of the fundamental 

 spicule on which the characteristic skeletal element, the desma, of the 

 lithistid sponges is built up. 



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