364 ORDER CHOANO-FLAGELLATA. 



exceeding the length of the body ; contractile vesicles two in number, 

 posteriorly located ; endoplast spherical, subcentral ; common mucilaginous 

 matrix or zoocytium exceedingly transparent, forming a more or less ex- 

 tensive film-like expansion on the surface of the water or over submerged 

 objects, containing from six to eight to as many as fifty or sixty or more 

 zooids. Length of zooids 1-3000". HAB. Fresh water. 



This very interesting form was obtained by the author so recently as July 1880, 

 in water containing Myriophyllum, and other aquatic plants, brought from the lake 

 in Kew Gardens. While detected in some few instances growing upon this vegeta- 

 tion, the more luxuriant colony-stocks were discovered forming faintly granular, film- 

 like expansions on the glass or the surface of the water after some days' isolation in 

 a shallow glass receptacle. Until the existence as an independent structure of the 

 entirely transparent or very faintly granular zoocytium was definitely determined, it 

 was presumed that the collared monads that excrete this element were colonies 

 only of a species of Monosiga such as M. sodalis, that had developed upon the 

 surface of a bacterial film or other foreign organic mucilage. The isolation of colony- 

 stocks and the registration of the constantly augmented dimensions of this zoocytium 

 pari passu with the increase in number of the contained zooids, speedily demon- 

 strated, however, the incorrectness of this first inference. The import of this film- 

 like excretion being thus determined, the close affinity of the type to Cienkowski's 

 genus Phalansterium was immediately recognized. Compared with that organism, 

 it at the same time exhibited several important features of distinction, the chief of 

 these being the well developed, in place of the rudimentary condition, of the terminal 

 collar and the exceedingly hyaline instead of coarsely granular mucilaginous 

 zoocytium. The collars in this type agreed essentially, in fact, in form and function, 

 with those of the several genera Monosiga, Codosiga, and Salpingocca previously 

 described. This rudimentary condition of the collar and accompanying coarsely 

 granular condition of the zoocytium in Phalansterium, and the transparency of this 

 zoocytial element in Protospongia, conjoined with a well-developed collar, are 

 correlations that evidently admit of a logical explanation. In allied forms possessing 

 similar well-developed collars it has been demonstrated by the author that all effete 

 matters are cast out within the discoidal area circumscribed by the base of the 

 structure, and hence in Protospongia they would be thrown out beyond the periphery 

 of the zoocytium, and could not possibly get entangled in its substance. In 

 Phalansterium, on the other hand, where the collar exists as a rudimentary structure 

 only, no such terminal liberation of the waste products can take place, but instead 

 of this are probably got rid of through the general peripheral surface, as occurs in 

 Rhipidodendron and Spongomonas, and further becoming, as in these genera, incor- 

 porated within the substance of the zoocytium. This interpretation is entirely 

 supported by the illustrations of the genus Phalansterium recently published by 

 Stein, in which this common mucilaginous matrix is depicted as enclosing uni- 

 formly distributed coarse granular corpuscles identical in appearance with those 

 that undoubtedly represent faecal rejectamenta in the two previously cited genera. 



The instability of contour and extreme plasticity of the constituent sarcode are 

 more marked in the zooids of Protospongia Haeckeli than in any other animalcule of 

 the Choano-Flagellate order so far examined. On the slightest disturbance the collars 

 and flagella are withdrawn, and an altogether irregular amoebiform aspect assumed, 

 as shown at a, a, a, a, in PI. X. Figs. 20 and 21. The binary fission of the zooids 

 during the assumption of a similar amoeboid state, as in Monosiga and Salpingoeca, 

 was frequently observed, as also their subdivision into larger or smaller sporular 

 elements, as at 22 a and 20 s of the same plate. The development of these sporular 

 bodies to the characteristic collared state was likewise traced, their initial condition 

 being that of simple uniflagellate monads, which, taking up a position in the 

 zoocytium adjacent to the adult zooids, as shown at PI. X. Fig. 22 bb, speedily 

 acquire the parent form. The establishment of new colony-stocks by similar but 

 single monoflagellate germs was likewise witnessed. In its initial condition such a 



