172 NATURE AND AFFINITIES OF THE SPONGES. 



differentiated metazoic tissue-elements they may be said, in fact, to typify 

 in both form and function. 



It is undoubtedly through the stimulus received and transmitted by 

 the cytoblasts that the characteristic contractions and expansions of the 

 oscula, and other portions of the sponge-body, are accomplished. Strictly, 

 however, these elements perform a more general function than that of 

 simple nerve-ganglia, they being in addition the direct motor agents in 

 the contraction and expansion of the general cytoblastema, and thus fulfil- 

 ling the part of both nerve- and muscle-fibres. The independent existence 

 and characteristic aspect and functions of the cytoblasts were first pointed 

 out by Lieberkuhn,* his observations being since abundantly confirmed by 

 the independent researches of Carter, H. James- Clark, F. E. Schulze, and 

 many other investigators, including the author. 



At first sight, the connection between these amoebiform cytoblasts and 

 the more essential collar-bearing zooids is scarcely obvious and has not as 

 yet, apparently, been even so much as suspected by any other authority. 

 To arrive at a comprehension of their true significance it is only necessary, 

 however, to refer to the life and developmental phenomena presented by 

 the independent collared monads and Flagellate Infusoria generally. Here, 

 both the primary and terminal conditions of the typical flagellate zooids 

 are frequently characterized by the exhibition of a similar amoeboid aspect, 

 as is abundantly shown in the accounts and illustrations given of the life 

 and developmental phases of the genera Codosiga, Salpingceca, Monas, 

 Oikomonas, Euglena, EiUreptia, Heteromita, and a host of other forms 

 described in this volume. The capability of the adult collared and flagel- 

 liferous spongozoa to take upon themselves a similar amoebiform character 

 has been observed repeatedly by the author, as illustrated in a large number 

 of figures contained in PI. VIII., which is confirmed by the observations of 

 Carter, F. E. Schulze, and other recent investigators, including even Pro- 

 fessor Haeckel himself.t The amoebiform elements of a sponge-stock 

 cannot therefore be consistently regarded even as independent structures. 

 To all intents and purposes they are the mere larval or metamorphosed 

 phases of the typical and essential collar-bearing zooids; the distinctive 

 title of cytoblasts, as here adopted, being conferred upon them only as a 

 matter of convenience. Where, as frequently occurs, the amoebiform 

 bodies are of comparatively colossal size, the coalescence of a greater or 

 less number of the ordinary cytoblasts has undoubtedly taken place, the 

 phenomena in this instance being directly comparable with the building 

 up of the huge amoebiform " plasmodia " of the Mycetozoa, or with the 

 coalescence of a number of metamorphosed amoebiform elements as ex- 

 hibited by the simple Flagellate types Heteromita uncinata and H. amyli. 

 The import of the amoeba-like masses thus constructed is likewise in all 

 instances identical ; each such aggregate mass ultimately producing, by 



* " Ueber das contractile Gewebe dcr Spongien." Miiller's 'Archives,' pp. 74-86, 1S67. 

 t See ' Monograjili of the Calcispongicv,' Taf. 25, fig. 6: " Vier Geissckcllen welchc sich in 

 amoeboide Zellcn vcrwandelt haben." Also, PI. VIII. Figs. 32 to 35 of this work. 



