114 M. E. Hackel on the Organization of Sjjoiiges, 



of organic substance (niodilied ])rotoplasni). In many Calei- 

 spongiffi the carbon-compound takes so considerable a share in 

 the formation of the skeletal parts, that the latter, after the 

 extraction of the carbonate of lime by muriatic acid, remain 

 quite unchanged in form and size, Avhilst only a slight residue 

 of molecular calcareous dust is left after calcination. 



The forms of the skeletal parts or spicules in the Calci- 

 spongije are, as is well known, by no means so multifarious 

 as in the Silicispongii:e. Only the four following fundamental 

 forms occur, with various moditications : — 1. Simple spicules 

 (linear, cylindrical, or fusiform), frequent. 2. Two-limbed 

 spicules (forked or hooked), very rare. 3. Tliree-limbcd or 

 triradiate spicules (with equal or unequal limbs and with 

 equal or unequal angles), by far the most frequent, and at the 

 same time the form most characteristic of the Calcispongiaj. 

 4. Four-limbed or quadriradiate spicules (the fourth ray of 

 Avhich usually projects freely into the canal-system). The 

 different moditications of these four fundamental forms, Avhich 

 have hitherto occupied the attention of the observers of the 

 Calcispongiffi more than all the rest of their organization, Avill 

 be conq)letely described in my monograph. 



That the Calcis])ongia3 of all living s])onges are most nearly 

 allied to the corals, may be inferred in the first place even 

 from the calcareous nature of the skeleton in the two groups. 

 But to this may be added very interesting homologies in the 

 special differentiation of the canal-system in the most highly 

 developed forms of the Calcispongi^, which in part directly 

 approach the sim])ler forms of corals even by the formation 

 of antimera*. We may therefore be allowed, in conclusion, 

 to glance at the stej)s in the evolution of tlie canal-system 

 in the Calcispongia3. 



At the root of the Avhole system (or, what is the same thing, 

 of the genealogical tree) of the Calcispongiai stands the remark- 

 able ProsT/cnm, the little calcareous sponge whose canal-system 

 c(msists merely of a stomachal cavity Avith a mouth-opening. 

 Next to this comes Oli/nthus, a simple" person " with stomach 

 and mouth-opening, but the stomachal wall or body-wall of 

 which is permeated by perfectly simple pores. These cutaneous 

 pores are simple breaches in the parenchyma, which perforate 



* IlJickel applies the term "«»</»?«•«" to the "homotypic organs" of 

 Bronu — that is to say, to those segments of the body, placed side by side, 

 of which each contains " all or nearly all the essential parts of the body 

 uf the species." The segments of the Radiate animals, as indicated in 

 the text, fnruish the most striking examples of this mode of formation. 

 Where the repetition of parts occnrs in eonsecntive segments (as in the 

 Annnlosa), these are called " metamera " by Hackel. — \V. S. D. 



