FORAMINIFERA AND RADIOLARIA. 7T 



Similar threads, passing off from the outermost zone, through the mul- 

 tiple ranges of marginal pores, would doubtless act as pseudopodia. 



470. Now this plan of growth is so different from that previously 

 described, that there would at first seem ample ground for separating the 

 simple and the complex types as distinct species. But the test furnished 

 by the examination of a large number of specimens, which ought never 

 to be passed-by when it can possibly be appealed to, furnishes these very 

 singular results : 1st. That the two forms must be considered as speci- 

 fically identical ; since there is not only a gradational passage from one 

 to the other, but they are often combined in the same individual, the 

 inner and first-formed portion of a large disk frequently presenting the 

 simple type, whilst the outer and later-formed part has developed itself 

 upon the complex: 2d. That although the last- mentioned circumstance 

 would naturally suggest that the change from the one plan to another 

 may be simply a feature of advancing age, yet this cannot be the case ; 

 since, although the complex sometimes evolves itself even from the very 

 first (the 'nucleus,' though resembling that of the simple form, sending 

 out two or more tiers of radiating threads), more frequently the simple 

 prevails for an indefinite number of zones, and then changes itself in the 

 course of a few zones into the complex. No department of Natural 

 History could furnish more striking instances than are afforded by the 

 different forms presented by the Foraminiferal types now described, of 

 the wide range of variation that may occur within the limits of one and 

 the same species ; and the Microscopist needs to be specially put on his 

 guard as to this point, in respect to the lower types of Animal as to those 

 of Vegetable life, since the determination of form seems to be far less 

 precise among such than it is in the higher types. 



471. In what manner the reproduction of Orbtiolites is accomplished, 

 we can as yet do little more than guess; but from appearances sometimes 

 presented by the sarcode-body, it seems reasonable to infer that gemmules, 

 corresponding with the zoospores of Protophytes ( 244), are occasionally 

 formed by the breaking-up of the sarcode into globular masses; and that 

 these, escaping through the marginal pores, are sent forth to develop 

 themselves into new fabrics. Of the mode wherein that sexual operation 

 is. performed, however, in which alone true Generation consists, nothing 

 whatever is known. 



472. ARENACEA. In certain forms of the preceding family, and 

 especially in the genus Miliola, we not unfrequently find the shells 

 encrusted with particles of sand, which are imbedded in the proper shell- 

 substance. This incrustation, however, must be looked on as (so to 

 speak) accidental ; since we find shells that are in every other respect of 

 the same type, altogether free from it. A similar accidental incrustation 

 presents itself among certain ' vitreous ' and perforate shells; but there, 

 too, it is on usually a basis of true shell, and the sandy incrustation is 

 often entirely absent. There is, however, a group of Foraminifera in 

 which the true shell is constantly and entirely replaced by a sandy enve- 

 lope, which is distinguished as a 'test;' the arenaceous particles being- 

 held together only by a cement exuded by the animal. It is not a little 

 curious that the forms of these arenaceous ' tests ' should represent those 

 of many different types among both the ' porcellanous' and the ' vitreous ' 

 series; whilst yet they graduate into one another in such a manner, as to 

 indicate that all the members of this ' arenaceous 7 group are closely 

 related to each other, so as to form a series of their own. And it is 

 further remarkable, that while the Deep-sea dredgings recently carried 



