694 ORDER PERITRICHA. 



recently by Greeff, to be composed of distinct but closely approximated muscular 

 fibrillae. . 



The numerous species of the genus Zoothamnium may, for the purposes of 

 diagnosis, be conveniently separated into two groups, a and b, that shall include 

 respectively those in which the zooids of the same colony-stock resemble or differ 

 from each other in shape or size. In the polymorphic series the usually fewer and 

 larger zooids developed become separated from their pedicles, and after a short 

 migratory existence reattach themselves and lay the foundation of new colonies. 



A. ZOOIDS POLYMORPHIC. 



Zoothamnium arbuscula, Ehr. PL. XXXVII. FIGS. 1-8 AND 25. 



Bodies of two shapes and sizes, mostly conical-campanulate, others 

 spherical, grouped on a compound pedicle or zoodendrium which consists of 

 an upright main rachis, from the summit of which divaricate horizontally or 

 obliquely a greater or less number of attenuate, more or less dividing 

 secondary branches ; the more abundant and smaller campanulate zooids 

 thickly distributed upon these secondary ramifications, interspersed here and 

 there, mostly at the axillae, with the larger, comparatively rare, spheroidal 

 animalcules ; main rachis very thick, highly elastic, contracting in a zigzag 

 form. Length of the ordinary zooids 1-430", height of the entire zooden- 

 drium 1-4". HAB. Fresh and salt water. 



The colonies of this species often occur in such abundance on the front glasses 

 of the tanks of marine aquaria as to obscure clear vision, constant and considerable 

 labour being necessitated in their removal. Examined separately, each colony-stock 

 ma.y be compared to a minute crystalline standard fruit-tree, of which the ordinary 

 campanulate zooids may be likened to the leaves and the spheroidal or reproductive 

 units to the fruit. Except for the presence of the larger reproductive zooids, these 

 tree-like zoodendria bear no inconsiderable resemblance to those of Carchesium foly- 

 pinum, such similarity probably giving rise to the circumstance of the last-named 

 species being accredited by some authorities with both a salt- and fresh-water habitat. 

 That Zoothamnium arbuscula flourishes indifferently in both these media is at the 

 same time a well-authenticated fact, both salt- and fresh-water examples having been 

 recently remitted to the author by Mr. Thomas Bolton. The representation of the 

 contractile pedicle, given at PI. XXXVII. Fig. 25, is derived from the delineations 

 of the species given by Mr. H. E. Forrest in the ' Midland Naturalist ' for May 

 1879. While succeeding in detecting an obscure longitudinal striation of the central 

 muscular fibre, the author has been unable to define the existence of transverse strise 

 as depicted in this figure, neither does such a differentiation of this element appear 

 to have been recognized by any other investigator. The developmental phenomena 

 of the larger spheroidal zooids, also reproduced from Mr. Forrest's drawings, in 

 Figs. 6 and 7 of the same plate, accord with those hereafter described of Zoothamnium 

 alternans. 



Zoothamnium niveum, Ehr. PL. XXXVII. FIGS. 13 AND 14. 



Bodies of two sizes, the majority elongate-campanulate ; main stem of 

 the pedicle prolonged, sinuous or spiral, giving off short alternate or almost 

 verticillate branches, the spherical zooids of large size, situated near the 

 bases of the primary branches. Length of the ordinary zooids 1-210". 



HAB. Salt water. 



