156 



NATURE 



\yune 24, 1875 



to the investigation of the Infus6ria, Haeckel must be mentioned 

 as the one who has brought the greatest amount of evidence to 

 bear on the question of their unicellularity. In a very elaborate 

 paper which has quite recently appeared,* and which is remark- 

 able for the clearness and logical acuteness with which the 

 whole subject is treated, Prof. Haeckel, resting mainly on the 

 observations of others, and partly also on his own, argues in favour 

 of the unicellularity of the Infusoria from the evidence afforded 

 both by the phenomena of their development and by the struc- 

 ture ot the mature organism. He confines himself chiefly to 

 the Ciliata — which, indeed, he regards as the only true Infu- 

 soria — while he considers the unicellularity of the Flagellata as 

 too obvious to require an elaborate defence. The value of this 

 paper will be obvious from the analysis of it which I now pro- 

 pose to give. 



In stating the argument derived from development, Haeckel 

 does not accept as established the alleged sexual reproduction of 

 the Infusoria, and he believes it safest to regard as non-sexual 

 *' sporee " the bodies {Keimkiigeht) which result* from the break- 

 ing up of the nucleus, and which Balbiani regarded as eggs. 



These bodies consist of a little mass of protoplasm usually 

 destitute of membrane, and including a nucleus within which 

 one or more refringent granules admitting of comparison with 

 a true nucleolus may sometimes be witnessed — characters which 

 are all those of a simple genuine cell. From this spore the 

 embryo is developed by direct growth and differentiation ot 

 parts ; but however great may be the differentiation, there is 

 never anything like the formation of a tissue. 



The development of the Infusoria is thus entirely in favour of 

 the unicellular theory. This theory, however, is just as strongly 

 supported by the study of their mature condition ; and here 

 Haeckel gives an admirable exposition of the structure of the 

 true or Ciiiate Infusoria. 



The parts which are common to all Ciliata and which first 

 differentiate themselves in the ontogenesis or development of the 

 spore, are the cortical layer, the medullary parenchyma, and the 

 nucleus, which is situated on the boundary between the two. 

 The differentiation of the protoplasm of the naked spore into a 

 clearer and firmer cortical substance, and a more turbid, granular, 

 and softer medullary substance, corresponds entirely with what 

 we see in the parenchyma cells of higher animals. These two 

 products of differentiation are designated by Haeckel "exoplasm" 

 and "endoplasm." 



The exoplasm is originally a perfectly homogeneous and struc- 

 tureless, colourless hyaline layer distinguishable from the turbid 

 granular soft protoplasm of the internal body mass, by containing 

 in its composition less water, by absence of included granules, and 

 by Us high independent contractility. All the mobile appendages 

 ot the body, trie cilia, bristles, spines, hairs, hooks, &c., are 

 nothing but structureless extensions of this exoplasm and partici- 

 pate in its contractility. In this respect they entirely correspond 

 to the cilia and llagelia; of the cells which form the ciliated epi- 

 thelium ot multicellular animals. 



In many Ciliata we find this cortical layer or exoplasm itself 

 subsequently differentiated into distinct strata. In the most 

 highly differentiated Ciliata lour layers may be distinguished as 

 the result of ths secondary differentiation of the exoplasm. 

 These are : (i) the cuticle layer, (2) the cilia layer, (3) the 

 myophan layer, (4) the trichocyst layer. 



The ciUicle is nothing but a lifeless exudation from the surface. 

 In the majority of Ciliata there is no true cuticle, and in those 

 which possess it, it presents itself under various forms, as seen 

 in the thin, chitine-iike, hyaline homogeneous pellicle of Para- 

 mKCum and Trichodina, the outer elastic layer ot the stem o£ the 

 Vorticellinre, the protective sheath of Vaginicola, the chitin-like 

 cases of the Tiaiinnodeaa and Codonellida^, the beautiful lattice- 

 like siliceous shells of the Uictyocystidae, and many other shells, 

 case?, and shitll-like protections.t 



* Haeckel, "Zur Morphologie der Infusorien." Jenaische Zeitschr., 

 Band vii. heft 4, 1873. 



t In the same number of the Zeitschrift, Haeckel (" Ueber einige neue 

 pelagische Infusonen ") describes some highly interesting Infusoria which 

 spend their lives in the open sea and are distinguished by the possession of 

 variously formed shells. His attention was first directed to them by finding 

 their elegant empty shells in the extra-capsular sarcode of Radiolarise. 

 These pelagic Infusoria appear to belong to two different groups, which stand 

 neareit to the TiniinnodeEe of Claparede and Lachmann. He designates 

 them as Dictyocyitida: and Codoneliidte. 



The family of the Dictyocystida; is based on Ehrenberg's Dictyocysta, 

 and is characterised by the possession of a siliceous perforated laince-like 

 shell so closely resembling that of many Radiolarise, that Haeckel at first 

 mistook it for the shell of one of these. The shell is in all the species bell- 

 shaped or helmet-shaped, and the body of the animal, which is fixed to the 



The cilia layer occurs in all Ciliata ; it lies immediately beneath 

 the cuticle where this is present, and the whole of the cilia and 

 other mobile appendages are its immediate extensions. These 

 must therefore perforate the cuticle or its modifications when 

 such protective coverings exist. 



The 7nyophan layer is identical with that which most authors 

 describe as a true muscular layer. It has been demonstrated in 

 most of the Ciliata. It appears as a system of regular parallel 

 fine strice in the walls of the body, and in the Vorticellida; occu- 

 pies also the axis of the stem, where it forms the characteristic 

 ' ' stem-muscle " of these animals. There can be no doubt that 

 these striae represent contractile fibrils, which, by their contrac- 

 tion, effect the various form changes of the animal. They are 

 thus physiologically analogous to muscles. From a morphological 

 point of view, however, we must regard them as only differen- 

 tiated protoplasm filaments. In the morphological conception 

 of true muscle, its cell nature is absolutely indispensable. The 

 so-called muscle-fibrils of the Infusoria never show a trace of 

 nucleus. They can be viewed only as parts of a cell due to the 

 differentiation of the sarcode molecules of its protoplasm ; and 

 as they are thus only sarcode filaments, Haeckel designates them 

 by the term "myophan," as indicating a distinction from proper 

 muscle. 



The trichocyst layer occurs also in many Infusoria, but not in 

 all. It is a thin stratum of the exoplasm lying immediately on 

 the endoplasm, and including in certain species the trichocysts. 

 The presence of these bodies, which possess a striking resem- 

 blance to the thread-cslls of the Ccelenterata, has, as we have 

 already seen, been urged as an argument in favour of the multi- 

 cellularity of the Infusoria. But, as Haeckel argues, no evidence 

 of muliicellularity can be derived from this fact. The thread- 

 cells of the Ccelenterata are themselves the products of a cell, 

 and we often find many of them originating in a single formative 

 cell quite independently of the nucleus ; the formative cell may 

 in this respect be compared with the entire body of the 

 Infusorium. 



It is the endoplasm, or internal parenchyma of the Infusoria 

 that has given rise to the most important differences of opinion, 

 and in his account of this part of the Infusorium-organism Haeckel 

 chiefly directs his criticism against the views advocated by 

 Claparede and Lachmann, and by Greeff. 



These authors, as we have already seen, compare the Infu- 

 soria with the Ccelenterata, and regard the endoplasm not as a 

 real part of the body, but merely as the contents of the alimentary 

 canal — as a sort of food mash or chyme contained in a spacious 

 digestive cavity whose walls are at the same time stomach wall 

 and body wall, and into which the mouth leads by a short gullet. 

 As Haeckel urges, however, it needs only a correct conception 

 of the intestinal cavity throughout the animal kingdom and of its 

 distinction from the body cavity, in order to show the tintenable- 

 ness of this position. The main point of such a conception lits 

 in the fact that the intestinal cavity and all extensions of it 

 (gastro-vascular canals, &c.) are always originally clothed by the 

 endoderm or inner leaflet of the blastoderm, while the body cavity 

 is always formed on the external side of the endoderm, and 

 between this and the ectoderm or outer leaflet of the blastoderm. 

 The body cavity and intestinal cavity of animals are thus essen- 

 tially different ; they never communicate with one another, and 

 always arise in quite different ways. 



Again, the contents of a true intestinal cavity consist only of 

 nutritious matter and water, in other words, of chyme ; while 

 the fluid which fills the body cavity is never chyme, but is 

 always a liquid which has transuded through the intestinal wall, 

 and which may be called chyle, or blood in the wider sense of 

 the word. 



Haeckel has thus taken, I believe, the true view of the intes- 

 tinal and body cavities of animals. He had already advocated 

 it in his work on the Calcareous Sponges. It necessarily in- 



fundus of the bell, and can be projected far beyond its margin, lias a wide 

 funnel-shaped peristome on whose edge are two concentric wreaths ef strong 

 cilia. He describes four species, distinguishing them by characters derived 

 from their siliceous latticed shell. 



The family of the Codonellidae, based on the genus Codouella, Haeckel, is 

 also provided with a bell-shaped case, but this, instead of being formed of a 

 siliceous lattice work, consists of a chitine-like organic membi-ane, through 

 which siliceous particles are scattered. The family is, however, chicHy 

 characterised by the peculiar form of its peristome. This is funnel-shaped 

 and provided on its margin with a thin collar-like expansion. The free edge 

 of this collar is serrated, and each tooth carries a stalked lobe of a piriform 

 shape, regarded by Haeckel as probably an organ of touch. At some dis- 

 tance behind the circle of piriform lobes is situated a ring of long, strong, 

 whip-like cilia, which form powerful swimming organs. The three species 

 described are distinguished by the form of their chitinous cases. 



