FAMILY OPALINID&. 557 



anteriorly and in a like manner enable their possessors to effect a tenacious hold 

 upon the intestinal walls of their selected hosts. A yet more remarkable peculiarity 

 suggestive of affinities in this same direction is, however, exhibited by certain species 

 of the genus Anoplophrya, in which, as exemplified most conspicuously in the type 

 presently described under the title of Anoplophrya prolifera, multiplication is effected 

 through the multiple segmentation of the posterior region of the body in a manner 

 closely resembling what obtains in the characteristic " proglottid " condition of the 

 tapeworms or Cestoid ea. 



A very marked diversity of internal structure subsists between the several generic 

 forms of the present family group, all of which were formerly included in the 

 single genus Opalina. Thus, while in Opalina proper, as here defined, no trace has 

 yet been detected of the existence of a contractile vesicle, in the two generic forms 

 Anoplophrya and Hoplitophrya such an organ exists not only in a multiple form, but 

 more usually exhibits a regular serial plan of disposition, which may be regarded as 

 pretypifying the extended canal-like development of this element that obtains in the 

 allied genus Hoplitophrya and other higher Ciliata. The modifications observable in 

 the development of the endoplast or nucleus among the generic members of the 

 Opalinidae are also very extensive. With the majority of species this structure is found 

 to present a simple linear or band-like aspect, while in others it takes the form of one 

 or two simple ovate bodies. The most abnormal formula of endoplastic development 

 is, however, met with in the typical member of the genus Opalina, O. ranarum, 

 originally supposed to be entirely destitute of any such structure, but in which, through 

 the recent investigations more especially of Engelmann* and Ernst Zeller,f a perfect 

 multitude of minute spheroidal endoplasts or nuclei, each with a contained endo- 

 plastule or nucleolus, are shown to exist. Such multinucleate infusorial forms are 

 not inconsistently held by some authorities to possess a sound claim for acceptance 

 as multicellular organisms, and while not eligible for admission into the ranks of 

 the typical Metazoa, it would seem questionable whether they might not be appro- 

 priately relegated to such an intermediate zoologic series as the " Mesozoa," 

 proposed by Ed. van Beneden for the reception of the singular endoparasitic genus 

 Dicyema of Kolliker, and which is actually treated by Claparede and Lachmann, 

 in their ' Etudes sur les Infusoires,' as a form most closely allied to the genus 

 Opalina. It has been more recently shown by Van Beneden, however, that Dicyema 

 is composed structurally of a central fusiform or subcylindrical axial cell around 

 which is superimposed a single layer of smaller, flattened, peripheral or cortical 

 cells, each possessing well-defined limits and a central nucleus, and having their 

 external surface covered with long, fine, vibratile cilia. As already suggested at 

 page 481 of the present volume, it would seem just possible that this long axial cell 

 in Dicyema is represented or pretypified among the Opalinidse by the abnormally 

 developed axial element or so-called nucleus that is met with in the genera 

 Anoplophrya and Hoplitophrya. The homoplastic resemblance subsisting between 

 the multinucleate Opalina. and the larval phases or " Planulae " of the Ccelenterate 

 sub-kingdom has been also referred to at page 480 of the present volume. 



Taken in their entirety, the group of the Opalinidae can by no means be 

 regarded as representing the most lowly organized section of the Infusoria-Ciliata, 

 as has been maintained by Stein and some other authorities, for while the oral 

 aperture, by reason of the special surroundings, has either remained undeveloped 

 or become entirely obliterated, other structural features equal or excel in complexity 

 those encountered in any other infusorial types, and point indirectly, as just intimated, 

 to a relationship beyond the pale of the Protozoa. 



Through the Dicyemida and Orthonectida, a direct relationship would appear to 

 be established between the Opalinidae and Scolecida, but it yet remains to be 

 determined by what ontogenetic process these several mouthless and essentially 

 endoparasitic organic groups have been primarily produced. With many, the inter- 

 pretation that they are retrograde offshoots only of the ordinary stomatode 



* ' Morphologische Jahrbuch,' Bd. i. Heft, iv., 1876. 



t 'Zeitschrift fiir Wissenschaftliche Zoologie,' I3d. xxix. Heft, iii., 1877. 



