8 10 MICROSCOPIC FORMS OF ANIMAL LIFE 



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 depart- 

 ment 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. 1 



In what manner the reproduction of Orbitolites 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 proto- 

 phytes, 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. 



Arenacea. In certain forms of the preceding family, and espe- 

 cially in the genus Mitiola, we not unfrequently find the shells en- 

 crusted 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 usually on a basis of true shell, 

 and the sandy incrustation is often entirely absent. There is, how- 

 ever, a group of Foraminifera in which the true shell is constantly 

 and entirely replaced by a sandy envelope, 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 ' 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 down to depths of from 1,000 to 2,500 fathoms 

 have brought up few forms of either ' porcellanous ' or ' vitreous ' 

 Foraminifera that were not previously known, they have added 

 greatly to our knowledge of the ' arenaceous ' types, the number and 

 variety of which far exceed all previous conception. These have 

 been systematically described by Mr. H. B. Brady, F.R.S., 2 whose 

 researches have led him to believe that the long- established division 



1 For further information on the subject of Orbitolites see the Author's account 

 of the genus in the reports of H.M.S. Challenger. Mr. H. B. Brady in his ' Challenger ' 

 Report (p. 224) describes a remarkable allied type from the Southern Ocean 

 Keramosphcera Murrayi in which the test is spherical, and the chambers are 

 arranged in concentric layers. 



2 See his important report on the Foraminifera dredged by H.M.S. Challenger 

 (1884), illustrated by 116 plates. A large number of deep-sea forms has lately been 

 described by Dr. A. Goes, from the dredgings of the Albatross; see Bull.Mus. Comp. 

 Zool. xxix. (1896). 



