GENUS ZOOTHAMNIUM. 695 



The Zoothamnium plumosum of Dr. Strethill Wright,* and also the Zoothamnium 

 spirale of Mr. P. H. Gosse t must be regarded as identical with this form. The 

 lower branches are frequently entirely bare, the animalcules apparently dying off as 

 the zoodendrium becomes distally extended, while the upper portion, bending 

 under the weight of the living zooids, assumes an ostrich-plume-like aspect. 

 The delineation of this species, given in the accompanying plate, is derived 

 from an example dredged attached to a Sertularian zoophyte from a depth of 

 fifty fathoms, off the coast of Falmouth in July 1879, representing one of the several 

 interesting infusorial types obtained by the author in connection with the summer 

 excursion of the Birmingham Natural History Society. As first figured and described 

 by the author in the ' Midland Naturalist' for March 1880, this form was included 

 as a variety only of Zoothamnium alternans. A further acquaintance with the more 

 important modifications of the last-named species has, however, determined the 

 recognition "of Z. niveum as a distinct type. 



Zoothamnium alternans, C. & L. 

 PL. XXXVI. FIGS. 22 AND 23, AND PL. XXXVII. FIGS. 20-24. 



Bodies of various sizes, mostly subpyriform or fig-shaped, a few of 

 the larger ones spheroidal ; peristome thick, widely everted, its front 

 border in contracted examples puckered or plicate; cuticular surface 

 finely striate transversely ; zoodendrium consisting of an erect and basally 

 very thick main rachis, from which are given off, at sometimes opposite 

 but mostly alternate intervals, a greater or less number of but rarely 

 subdivided primary branches, upon which are borne at evenly separated 

 intervals the smaller and larger pyriform zooids, the still larger sized 

 spheroidal zooids situated usually at the axes or junctures of the branches 

 with the main stem ; the external surface of the compound pedicle finely 

 striate or annulate throughout. Length of smallest pyriform zooids 1-555" 

 to 1-425", of the largest pyriform and subspheroidal zooids 1-200". 



HAS. Salt water. 



This species, while first discovered by Messrs. Claparede and Lachmann on the 

 Norwegian coast, was also met with by Greeff at Ostend, and has been frequently 

 obtained by the author, both in the Channel Islands and on the Devonshire coast, 

 attached to sea-weeds and to the polyparies of Sertularia and other littoral Hydrozoa. 

 It has been previously mentioned that the dendritic colonies of Zoothamnium arbuscula 

 present in their general mode of growth a fanciful resemblance to a symmetrically 

 grown standard fruit-tree ; adopting a like simile in the present instance, Z. alternans^ 

 in its most luxuriantly developed state, may be compared to a carefully trained 

 espalier. Examples taken from widely separated or even contiguous localities 

 exhibit nevertheless a considerable latitude of variation, in some the side branches 

 being much longer than in others, while the zooids attached to these branches are 

 as often oppositely as alternately disposed. The chief illustration given of this 

 species, PI. XXXVII. Fig. 20, reproduced from Greeff, J represents an example in 

 which the branches are very long, and the former formula of arrangement pre- 

 dominates, while at Fig. 2 1 is a colony, from the author's sketch-book, in which the 

 branches themselves are reduced to their minimum, and represented by one or at 

 the most only two zooids ; between these two extremes every gradation of develop- 

 ment may be found. The striation of the cuticular surface of the zooids, as also the 

 annulation of the branching pedicle, is likewise subject to individual variation, being 



* Pritchard's 'Infusoria,' p. 595, 1860. t 'Tenby,' pi. iv. fig. b, 1856. 



t " Untersuchungen iiber die Naturgeschichte der Vorticellen," ' Wiegmann's Archiv,' xxxvii 

 1871. 



