52 A. T. MASTERMAN [januaey 



masses of matter, and hence have similar stereometric relationships. 

 Here the analogy ends. 



Haeckel's classification of animal symmetry has not received the 

 amount of recognition it deserves from zoologists, partly because 

 it is encumbered with a copious and somewhat unwieldy nomen- 

 clature. Jaeger's text-book of zoology may be noted as one of the 

 very few which give anything like an adequate discussion of the 

 subject. 



The other classic work dealing with the subject is Spencer's 

 " Principles of Biology " (vol. ii. chaps, vi. and xiv.), which appeared at 

 about the same time as Haeckel's work. It is difficult to understand 

 why his classification of symmetry has not been more widely adopted, 

 though it is regrettable that a classification equally applicable to 

 both plants and animals, if anything rather more to the latter, should 

 be more or less hidden in a chapter on the Morphological Differentiation 

 of Plants. His classification, differing little from that proposed below, 

 is followed by a characteristic attempt to show the inter-relation 

 between the symmetry of an organism and its environment. Though 

 written over thirty years ago, there is scarcely a sentence that does 

 not apply with equal force at the present time. His three types 

 of spherical, radial, and bilateral symmetry are equivalent to the 

 centro-symmetry, axo-symmetry, and piano-symmetry, the first three 

 types instituted below. The justification for alteration in the nomen- 

 clature will be there stated. 



Amongst smaller works we may mention the essays of Dr. Amans, 1 

 dealing with the form of animals exhibiting aquatic locomotion. He 

 follows up the " ovoid " form from the sphere through circular, 

 elliptical, unisymmetrical, and asymmetrical ovoids, the bilaterally 

 symmetrical ovoid being correlated with the highest form of locomo- 

 tion. He thus recognises the great importance, previously pointed 

 out by Spencer, of the form of locomotion in determining the form 

 of symmetry. 



Haeckel, twenty years after his former work (in 1887), returned 

 to the question of animal symmetry in his " Eadiolaria." 2 He points 

 out that text-books such as those of Claus and Sachs give an 

 insufficient treatment of the subject, and whilst recognising the 

 difficulty of the nomenclature, he adheres to the principles formerly 

 stated. He here gives four principal types of ground-forms which 

 correspond to those of Spencer; and the first three are very nearly 

 equivalent to the three types proposed below, with centres composed 

 of a point, a straight line, and a plane respectively. His main sub- 

 divisions, the Homaxonia and Polyaxonia in the first group, and 

 Monaxonia and Stauraxonia in the second group, do not appear to me 

 to differ sufficiently in kind nor in degree to warrant the subdivisions, 



1 Comptes Rcnchis, cv. 1887, and Ann. Sci. Nat. vi. 

 2 Challenger Report, vol. xviii. 



