400 ORDER FLAGELLATA-EUSTOMATA. 



described in this treatise, was completely ignored by the talented staff of the ' Chal- 

 lenger' expedition, its representatives indeed, when encountered, as in the present 

 instance, being unrecognized and referred to the vegetable series. It is to be hoped 

 that the opportunities for acquiring a further knowledge of the morphology and 

 distribution of the hitherto little known or altogether unchronicled pelagic Infusoria 

 will not be neglected in the future scientific expeditions of a like kind that may be 

 organized by this country. It may be confidently anticipated that in this field 

 alone new and interesting forms remain to be discovered, that surpass in number and 

 variety the by no means inconsiderable series that are already known to science. 



GENUS II. LEPTODISCUS, Hertwig. 



Animalcules free-swimming, more or less flattened or discoidal, having, 

 as in Noctiluca, a hyaline investing pellicle and an internal, radiating or 

 reticulate, protoplasmic layer ; oral aperture situated at the base of a 

 tubular pit-like excavation ; a long, lash-like, vibratile flagellum issuing from 

 the entrance to the adoral fossa ; no supplementary tentaculate appendage ; 

 endoplast conspicuous ; no contractile vesicle as yet detected. 



HAB. Pelagic. 



R. Hertwig,* the original discoverer of the single known member of this genus, 

 is inclined to regard it as the type of a family distinct from Noctiluca. The various 

 structural details, excepting for the absence of the transversely annulate tentaculate 

 appendage, correspond however so closely with those of the latter type that its 

 complete separation from it seems scarcely justified. 



Leptodiscus medusoides, Hertwig. PL. I. FIGS. 46-53. 



Body very much flattened, meniscoidal and orbicular, highly flexible, 

 thickest in the centre and gradually tapering towards the periphery ; oral 

 fossa subcentral, opening on the convex surface, descending obliquely 

 through the entire thickness of the body to the protoplasmic lining of the 

 opposite bounding membrane ; flagellum slender, vibratile, its length equal 

 to less than one-fourth of the diameter of the disc, inserted close within 

 the entrance to the oral fossa ; endoplast oval, subcentral, consisting of a 

 larger granular and smaller transparent region ; a number of minute, 

 spherical, oil-like globules immediately underlying the upper and convex 

 cuticular layer. Diameter of largest individual 1-4". 



HAB. Pelagic : Messina. 



The close resemblance of the adult zooids of this singular species to minute jelly- 

 fish (Medusae) is very remarkable, their large size, as represented without the aid of 

 magnifying power at PL I. Figs. 46 and 47 further supporting this analogy. Hertwig 

 figures as the supposed young of Leptodiscus some organisms that differ altogether 

 from the parent. These bodies, PL I. Figs. 52 and 53, are of more elongate 

 outline, flattened and somewhat cushion-shaped, with a central, transversely annular 

 constriction ; there is no flagellate or other appendage, the endoplast is central, 

 oval, and granulate throughout, and represents the point from whence the internal 

 protoplasmic reticulations chiefly radiate, as in the adult form. The mode of 

 natation in these hypothetic non-adult zooids is exceedingly remarkable, being 

 effected, according to Hertwig, by the alternate contractions of the internal proto- 

 plasmic radii on each side of the central annular constriction, this contraction in 



* " Ueber Leptodiscus medusoides ," ' Jenaische Zeitschrift,' Bd. xi., Nov. 1877. 



