INFUSORIA. 27 



and Claparede and Lachniann. The unicellular theory was first formally opposed to 

 that of Ehrenberg by Siebold in 1845. It was advocated also by Schleiden and 

 Schwann. Haeckel has pointed out that the life-history of an infusorian, even one of 

 the liighest, is only an epitome of the Ufe-history of a simple cell. Kent, in his Manual 

 of Infusoria (1880) holds "that all infusorial structures possess a unicellular morphologic 

 value only." It seems to the writer that most living students of the Invertebrata hold 

 this view. 



The bodies of the lowest members of the group (for example, 3Iastigamoeba 

 simplex) are composed of simple protoplasm, exhibiting little or no difference in 

 density between the inner and outer portions ; in fact, their shape is but slightly more 

 constant than certain Amoebce ; the possession of a ilagellum alone pointing out to the 

 observer that the object under the assisted eye is not a rhizopod. On the other hand, 

 those of the higher members of the group, as the Vorticellidae, Fig. 44, have clearly more 

 dense external layers, either of naked protoplasm or of the same surrounded by a 

 cuticle of formed matter. Haeckel has described four distinct layers present in the 

 bounding walls of the highest typical forms. The first is an outer, delicate, hyaline, 

 elastic membrane ; this in the sedentary, stalked species, extends down 

 as the sheath of the pedicle, while in others it takes part in the forma- 

 tion of the carapax, as in Euplotes. As secondary products also may be 

 mentioned the loricae of Cothtirnia and its allies (Fig. 21) ; again, m O^Vi- 

 rydium, it surrounds the zooids as a thick mucilaginous sheath, binding 

 thousands of individuals into colonies. The second is a highly contrac- 

 tile layer just beneath the cuticle, called the ciliary layer; from this arise 

 the cilia and their various modifications. The third, found only in the 

 highest Ciliata, is termed the muscular layer ; it is prolonged through the 

 stalk of J^orticella, giving that organ its eminently muscular function ; 

 it is highly develoj^ed in Stentor and in Spirostomum ; in the former the 

 fibrilliB are arranged longitudinally, in the latter they have a spiral disposition. The 

 fourth of these layers is that in which the trichocysts (rod-like bodies possessed by cer- 

 tain forms) are generated. 



The contents of the complex bounding wall, or ectosarc, consists of the more fluid 

 protoplasm of the organism. This transparent, colorless sarcode, the endosarc, contains 

 numerous minute dark-colored granules, food particles, oil globules, foreign bodies, and 

 an important body which requires special notice, the nucleus. This is usually a more 

 or less oval body, differing somtjwhat from the surrounding protoplasm in density, etc., 

 and in the fact that it is stained more readily by reagents, thus enabling the observer 

 to easily distinguish it. In many cases, particularly among the simplest Infusoria, 

 Huxley says, that "the 'nucleus' is a structure which is often wonderfully similar to the 

 nucleus of a histological cell ; but as the identity is not fully made out, it may better be 

 termed endoplast." While it has not been absolutely settled that this body is a true 

 nucleus, there does not appear to be any valid objection to the view which would 

 homologize the nucleus of the Protozoa and Metozoa. There are various sjjecial forms 

 of the nucleus: it is sometimes ribbon-like, or coiled into a short, loose spiral (Fig. 44, A), 

 or moniliform, that is, composed of nodular or oval bodies sejjarated by constric- 

 tions; this form occurs in Sph-ostomum ambiguum (Fig. 22). The nucleolus or en- 

 dop)histule, when present, is, in the spherical forms, enclosed within the nucleus, 

 •while in the sausage-shaped nuclei it is often outside, attached to the lateral wall. 

 It has been demonstrated beyond question that in the more complicated tyjies this 



