696 ORDER PERITRICHA. 



strongly marked in some instances and scarcely visible in others. In the two 

 colonies only of this species examined by Claparede and Lachmann, and in most of 

 those observed by the author, these striee and annulations were very conspicuous ; 

 while in those figured by Greeff the zooids are represented as perfectly smooth, 

 while the annulations of the branching pedicle are very few and far between. As 

 explained by its original discoverers, the somewhat abnormal alternate divarication 

 of the lateral branches, and more usually similar disposition of the zooids upon these 

 branches, are produced by the unequal growth of the two animalcules which result 

 from the primary process of longitudinal fission ; while one of these remains 

 stationary or produces but a short pedicle, the other passes on and after a short 

 interval repeats the process of subdivision. 



It is upon the larger spheroidal zooids, as first intimated by Ehrenberg with regard 

 to the similar individuals in Z. arbuscula, that devolves the mission of becoming 

 detached and laying the foundation of future colonies. These larger animalcules are 

 at first similar in shape and size to the smaller pyriform units, but taking no share in 

 the further extension of the branching pedicle, increase greatly in size, retaining for 

 a time their pyriform outline, and ultimately assuming their characteristic spheroidal 

 contour. This last-named stage arrived at, a circlet of cilia is developed posteriorly, 

 as in the migrant zooids of Vorticella, and the animalcule shortly becomes detached 

 from the parent stalk and wanders away in search of a suitable site for reattachment. 

 The abnormal thickness of the basal portion of the branching pedicle in both Zootham- 

 nium arbuscula and the present species is explained by the light of these develop- 

 mental phenomena, its greater calibre being the natural product of a zooid of so 

 much greater a relative size than those which contribute to its further prolonga- 

 tion. The development by these spheroidal animalcules of the posterior ciliary 

 circlet, or their detachment from the parent stock, as observed by the author, and in 

 part by Claparede and Lachmann, does not appear to have been seen by Greeff. 



Very recently, September 1880, the author has received through Mr. Thomas 

 Bolton, from the Aston Aquarium, examples of this species among which were 

 scattered one or two colony-stocks that bore supplementary zooids of an altogether 

 abnormal size and contour; these, as shown at PI. XXXVI. Figs. 22 a a and 23, 

 equalled in length the ordinary pyriform zooids, but were very slender five or six 

 times as long as broad while their cuticular surface was relatively coarsely annulate. 

 It is anticipated by the author that these attenuated zooids represent male units 

 destined to compass genetic union with the large subspheroidal and subsequently 

 detached animalcule of the normal colony-stocks. Phenomena substantiating this 

 anticipation have yet to be recorded ; even should this not be forthcoming, Zootham- 

 nium alternans will afford one of the most remarkable known instances of poly- 

 morphism among the Infusoria, in no other type being there three distinctly 

 differentiated zooidal forms. 



Zoothamnium Cienkowskii, Wrz. PL. XXXVII. FIG. 15. 



Animalcules of two sizes, but similar in shape, campanulate, attenuate 

 posteriorly ; cuticular surface smooth ; peristome-border thick, widely 

 dilated ; ciliary disc slightly elevated, the cilia surrounding it very 

 long and fine ; pedicle massive, dichotomously branched, supporting but 

 few zooids, at most seven or eight, but more usually only two or four, the 

 basal portion of the pedicle thickest, smooth, or finely striate longitudi- 

 nally, the remaining portion from the commencement of the branches trans- 

 versely wrinkled ; the main rachis slightly exceeding in height the length 

 of the larger zooids, the primary branches shorter than the secondary ones ; 

 contractile vesicle of extraordinary size ; endoplast band-like, curved, 

 transversely placed. Length of smaller zooids 1-665", f larger ones 

 1-500". HAB. Salt water. 



