and the Tlieory of Descendence. A21 



by which they are most strikingly distinguished from most 

 other organisms, is the extraordinary instability of the outer 

 form of the hody. It is this that renders their study so in- 

 structive in connexion with the problem of species. Every 

 systematist knows how great and decisive is the significance 

 of the external form in the distinction of species in almost 

 every class of animals ; indeed the great majority of species 

 are distinguished merely by more or less important difterences 

 in the details of the external form. In complete opposition to 

 this, the external form in the Sponges, and especially in the 

 Calcispongige, is so variable that it cannot be employed at all 

 for characterizing species, either in the natural or the artificial 

 system. What I have observed in this respect among the 

 Calcispongi^ exceeds all previous conceptions, and goes much 

 further than the wonderful variability of the external form in 

 the Fibrospongije, which have been indicated as quite extra- 

 ordinary by all recent spongologists, especially Oscar Schmidt. 

 A systematist who should adopt the external form alone as a 

 specific character in the case of Ascandra variahilisj Leucetta 

 iwimigenia^ or Sycandra compressa might at his pleasure di- 

 stinguish among the individuals of any one of these extremely 

 variable species from a single locality ten, twenty, or more 

 than a hundred species. 



It may perhaps seem still more remarkable that this ex- 

 cessive instability affects even the most important organs, such 

 as the stomachal cavity and the mouth. In very many natural 

 species we find side by side individuals with and without a 

 mouth. Among the Fibrospongiaj also the loss of both mouth 

 and stomach appears to be very frequent. This singular phe- 

 nomenon is probably to be explained by the fact that in the 

 Sponges (as in the parasitic worms, Crustacea, &c.) the mouth- 

 opening does not possess the same physiological importance 

 as in the higher animals. It becomes rudimentary and is finally 

 lost (Cestodea, Rhizocephala, lipogastric Sponges). The 

 quadruply different nature of the mouth in the Calcispongise is 

 also very variable. 



I have particularly described this remarkable multiformity 

 of the species of Calcispongise in the second volume, and elu- 

 cidated it by many figures. In the explanation of the plates 

 it is called jyolymorjjhosis, in contradistinction to the well- 

 known polymoiyhism of the Siphonophora and of many of 

 the higher animals. The latter is well known to be a product 

 of physiological division of labour. Polymorj)hosis^ on the con- 

 trary, is 2^ j^olymorphism xoitliout division of labour'^ its cause 

 is to be sought merely in adaptations to external conditions of 

 existence of quite subordinate importance. 



