No. 3.J MORPHOLOGY OF THE STENTORS. 519 



phylogeny — comparative anatomy and palaeontology — among 

 the Infusoria we have only the former. 



I regard the Heterotricha as showing more clearly than any 

 other group of Infusoria a phylogenetic history. They possess 

 one structure, the adoral zone, that we can consider absolutely 

 homologous from the highest to the lowest of the group. 

 Another structure, the peristome, which lies to the right of the 

 zone and in the higher forms is partly or almost wholly enclosed 

 by it, is also probably homologous throughout all the lower 

 forms of the group. The fact has already been noted (p. 502) 

 that in the Stentorina the peristome is replaced functionally by 

 the frontal field. These three structures, adoral zone, peristome, 

 and its successor, the frontal field, are more important than any 

 others in studying the comparative anatomy of these forms. 

 In passing from the lower to the higher Heterotricha, we find 

 that the peristome and zone take a more and more terminal 

 position, until in the Stentorina (CHmacostomum, Stentor, 

 Folliculina) it is completely apical (Fig. Ill, d, e, /). It is 

 possible, then, to arrange a series (which we may regard as 

 phylogenetic) based upon the position and development of the 

 adoral zone and frontal field (Fig. Ill, a-f). In this series the 

 forms in which these structures arc most strictly lateral in 

 position (Spirostomum, Blepharisma) will occupy the lowest 

 place, and the others (Condylostoma, CHmacostomum, Stentor) 

 be arranged according as the zone becomes more and more 

 terminal, and as the peristome (or its physiological equivalent, 

 the frontal field) increases in breadth and becomes more and 

 more circumscribed by the zone. 



This much being attainable by comparative anatomy, does 

 the ontogeny of one of the higher forms {e.g. Stentor) yield any 

 evidence for or against the arrangement given in Fig. Ill .? 

 In comparing the various stages in the evolution of the zone 

 and frontal field of the proximal zooid of 5. c(2rideiis (PI. XXIV, 

 Figs. 26-36) with the series of lower forms (Fig. Ill, a-d), we 

 see that the successive positions taken by the new organula in 

 question correspond quite accurately with the phylogenetic 

 development of these structures. An exception is seen in the 

 case of the peristome. This appears as a narrow band along 



