202 THE EVOLUTION OF THE METAZOA 



evolution of cnidae within the group of Cnidaria, can point 

 to the direction Anthozoa -> Scyphozoa -> Hydrozoa. This 

 progressive evolution can be observed in several elements. 

 First, in the general form of the capsule, we find in Antho- 

 zoa an oblong or rod-Hke form to be prevalent. It is only in 

 Scyphozoa, and especially in Hydrozoa, that it becomes 

 differentiated into several other external forms, particularly 

 into a spheric or into a pear-shaped form. Another develop- 

 ment can be observed in the w-all of the capsule. It is in 

 Anthozoa only that we find cnidae with a thin-walled 

 capsule and with a predilection for acid colouring substances; 

 they have been given a special name, i.e. that of spirocysts. 

 Earlier it was believed to be a sign of the primitive character 

 of these spirocysts that they do not show a continuity between 

 the wall of their capsules and the "thread." Will (1909), Weill 

 (1930), and recently C. E. Cutties (1955), however, have been 

 able to show that we nevertheless have here genuine cnidae, 

 even if the connection betw^een the capsule and the thread 

 still seems to be a tenuous one. 



A further progressive evolution can be observed in the 

 equipment and in the form of the thread. It has been used 

 as a basis for the systematization of cnidae by Weill (1930) and 

 by Carlgren (1945). A further subtle morphological and physi- 

 ological difference can be observed on and around the capsule 

 of the cnidae if we compare the anthozoan and hydrozoan 

 cnidae (cnidocil, lasso, various fibrous forms in the cytoplas- 

 mic body of the cnidoblast, the formation of the stem, etc.). 

 A conspicuous phenomenon is a progressive variety of the 

 cnidarian apparatus that can be observed in one and the same 

 species of Cnidaria, a fact w^hich has led to the notion of a 

 "cnidom." Such a differentiation can be further developed in 

 another direction, i.e. to a more regular and therefore to a 

 more effective grouping of cnidae into variously formed 

 batteries. Again this development has reached its climax in 

 Hydrozoa, especially in Siphonophora. In this connection we 

 also find a more and more distinct differentiation of the strictly 



