104 ZOOLOfJY SECT. 



perform one function, some another, and none of which exhibit 

 the independent life of the zooid of a protozoan colony. It cannot, 

 however, be said that there is any absolute distinction between a 

 colony of unicellular zooids and a single multicellular individual : 

 Proterospongia and Volvox approach very near to the border-land 

 from the protozoan side, and a similar approach in the other 

 direction is made by certain animals known as Mesozoa, which will 

 be discussed hereafter (Section IV.). Moreover, the Mycetozoa, the 

 plasmodia of which- are formed by the fusion of Amcebulse, the 

 nuclei of the latter remaining distinct and multiplying, are rather 

 non-cellular than unicellular. This point will also be referred to 

 at the conclusion of the section on Sponges (Section III.). 



In each division of the Protozoa we have found comparatively 

 low or generalised forms side by side with comparatively high or 

 specialised genera. For instance, among the Rhizopoda, there 

 can be no hesitation in placing the Lobosa, and especially Prot- 

 amoeba, at the bottom of the list, and the Radiolaria at the top. 

 Similarly, among the Mastigophora, such simple Flagellata as 

 Oikomonas (Fig. 53, 2 and 8) are obviously the lowest forms, 

 Noctiluca and the Dinoflagellata the highest. But whether the 

 Rhizopoda, as a whole, are higher or lower than the Flagellata is 

 a question by no means easy to answer. A flagellum certainly seems 

 to be a more specialised cell-organ than a pseudopod, and some 

 of the Mastigophora rise above the highest of the Rhizopoda in the 

 possession of a firm cortex and cuticle, and the consequent assump- 

 tion of a more definite form of body than can possibly be produced 

 by the flowing protoplasm of a Foraminifer or a Radiolarian. On 

 the other hand, the nucleus of the Radiolaria is a far more complex 

 structure than that of the Mastigophora ; and in Foraminifera, 

 Radiolaria, and Heliozoa the organism frequently begins life as a 

 flagellula, a fact which, on the hypothesis that the development of 

 the individual recapitulates that of the race, appears to indicate 

 that these orders of Rhizopoda are a more recently developed stock 

 than at any rate the lower Flagellata. These circumstances, and 

 the fact that Mastigamoaba might equally well be classed as a 

 lobose Rhizopod with a flagellum or as a Flagellate with pseudopods, 

 seem to indicate that the actual starting-point of the Protozoa was 

 a form capable of assuming either the amoeboid or the flagellate 

 phase. From such a starting-point the Lobosa, Foraminifera, 

 Heliozoa, Radiolaria, and Flagellata diverge in different directions, 

 the first four keeping mainly to the amoeboid form,^but assuming the 

 flagellate form in the young condition in the case of Foraminifera, 

 Heliozoa, and Radiolaria. 



The Choanoflagellata, Dinoflagellata, and Cystoflagellata are 

 obviously special developments of the Flagellate type along 

 diverging lines. 



