1 92 NATURE AND AFFINITIES OF THE SPONGES. 



existence a simply flagellate structural type. In their most characteristic 

 form, these reproductive bodies or cell-aggregates consist of a uniform 

 series of collared zooids, but by irregular growth one half may arrive 

 at or pass maturity in advance of the other, the product then being a 

 compound structure presenting a close correspondence with that phase of 

 development of the Metazoic ovum known as the amphiblastula. Since, 

 however, these bodies are in no way comparable with the Metazoic ovum 

 not being the product of the concourse of true sexual elements the above 

 likeness is simply homoplastic, and the body as a whole, consisting as it 

 does of an aggregation of numerous independent zooids, may be most 

 appropriately denominated a " swarm-gemmule." While no direct approach 

 to the production of a similar compound gemmule occurs among the 

 typical Infusoria-Flagellata, as at present known, something much akin 

 to it obtains in the protophytic type Volvox globator, which liberates 

 from its interior, free-swimming gemmules that take the form of spherical 

 aggregations of biflagellate daughter-cells. In their isolated state, on the 

 other hand, the svvarm-gemmules of the sponge-stock are directly com- 

 parable with the free-swimming subspheroidal colony-stock of the flagellate 

 Infusoria Synura, Syncrypta, and Uroglena, or with the attached subsphe- 

 roidal clusters of Codosiga and Anthophysa. 



In certain respects, as already pointed out at page 41 et seq., a very 

 remarkable and suggestive analogy in the direction of the Spongida is 

 furnished by the Protozoic group of the Myxomycetes or Mycetozoa. 

 Here we find the essential elements consisting primarily of independent 

 flagellate zooids possessing a spheroidal endoplast, contractile vesicles, 

 taking in solid nutriment, and presenting other characters in common with 

 the ordinary Flagellata. Passing their matured flagellate condition these 

 now assume an amoebiform condition and coalescing in large numbers, as in 

 the case of the Spongozoa, form a colossal amcebiform mass, the plasmo- 

 dium, not unlike the cytoblastema of a sponge with its amcebiform contents, 

 out of which by a species of encystment the characteristic fungus-like 

 sporangia are developed. These sporangia are to a considerable extent 

 comparable with the hibernating encystments or so-called " gemmules " 

 of Spongilla and other sponges, and subsequently, through the process 

 of segmentation, become resolved into innumerable minute spores, which 

 again give birth to a host of flagellate monadiform zooids resembling those 

 from whence they originally sprang. It is further remarkable and sugges- 

 tive of some distant affinity with the Spongida, that the network of fine 

 interlacing threads, or " capillitium," that frequently binds the enclosed 

 spores together, closely corresponds with the fine horny fibre of the 

 keratose sponge-series ; while in the substance of the outer wall or ' peri- 

 dium " of the sporangia of some Mycetozoa, such as Didymium nigripes and 

 D. serpula (see PI. X. Figs. 30 and 31), calcareous deposits resembling 

 sponge spicula are developed. 



It is clearly manifest that in a very singular manner, and to a marked 



