716 SYSTEMATIC HISTORY OF THE IJfFL'SOElA. 



at right angles to that of the original segments, but its plane of expansion is 

 at right angles to their plane. In each of these latter cases the entire ab- 

 normal specimen, therefore, forms a cross, — the interior here, of course, as 

 well as in those cases where the intervening growth does not assume any 

 definite outline, making but one uninterrupted cavity (III. 61). The omission 

 of the formation of a septum, however, can only be looked on as the primary 

 cause of the aberration, the curious change in the direction of the new growth 

 not necessarily following, as the figures, pi. 1. f. 9-11 {loc. c'lt.) seem to 

 prove (III. 62). 



The assertion that zoospores occur in this family is based upon the observa- 

 tions made by Mr. W. Archer on Docidium Ehrenhergii (Rails), and recorded 

 and figured in Proceedings Nat. Hist. Soc. Dublin, Febniary 1860 ; also Nat. 

 Hist. Review, July 1860. These observations, though unfortunately and 

 unavoidably not so full in their details as the interest of the case would lead 

 us to wish for, seem to warrant the assumption that the species of this family 

 may be occasionally propagated by zoospores, predicating of the family that 

 which seems to hold in the species in question {Docidium liJJirenheryii). Pedi- 

 astreae are of course not taken into account. Briefly, the phenomenon alluded 

 to is as follows (III. 46, 47) : — From beneath the base of one of the segments, 

 either one, two, or thi'ee (the latter rarely) lateral tubercle-hke projections 

 are formed, originating not from any portion of the segment itself, but from 

 an extension thereto produced between the inflated base and the sutural line. 

 When more than one is formed, they are usually opposite, but sometimes side by 

 side. A gradual elongation of the projection (or projections) then takes place, 

 the endochrome in the immediate neighbourhood becoming finely granular, 

 and filling what has now become an elongate lateral tube (or tubes) like the 

 finger to a glove, the remainder of the endochrome being as yet not much 

 altered, and the terminal clear space with the active granules being still in situ. 

 The endochi'ome within the lateral tube and in its immediate neighbourhood 

 now becomes segmented into a number of definitely bounded individuahzed 

 portions, which presently one by one emerge through the opened apex of the 

 lateral tube, and become associated together in an external cluster. The 

 remaining endoclirome now becomes drawn into bands, tm^ns bro^vn, and 

 speedily dies. The cluster of gonidia at the apex of the lateral tube now 

 appear to have become encysted each within its own special coat ; and the 

 green contents can be seen twisting backwards and forwards within the con- 

 fining membrane. After a time the contents emerge each from its cyst, by 

 rupturing it, and slowly swim away as pyriform or ovate ciliated bodies, — 

 as we apprehend, veritable zoospores. The author was entirely unacquainted 

 with their after-history ; but they resemble so much, in their appearance, 

 growth, and mode of escape from the parent-cell, the similar bodies in Cla- 

 dophora, &c., which are indubitable zoospores, that we imagine there can be 

 little question as to the nature and function of the bodies occurring in Doci- 

 dium. It will be noticed that this phenomenon is altogether distinct from, 

 and we believe in no way to be confounded with, that of the active molecular 

 movement of the ultimate granular particles of the endochrome alluded to at 

 pages 10 and 19 of the General History, — a circumstance which, indeed, some- 

 times accompanied the special one here described, in Mr. Archer's specimens, 

 but sometimes did not, and which is one of very general occui'rence under 

 other circumstances and in other cases, and has probably given rise to the 

 assumption, often made in our EngHsh books, that zoospores occur in the 

 Desmidiaceee. Xor is the production of zoospores here briefly described to 

 be in any way confounded M'ith the development of the parasitic plant Pijtkium 

 entophytum (Pringsheim), nor of any species of Chytridium (Braun). The 



