62 PROTOZOA 



the lapse of time and the pressure of superincumbent layers. 

 Some Foraminifera live on the sea bottom even at the greatest 

 depths, and of course their shell is not composed of calcareous 

 matter. Foraminifera may be obtained for examination by care- 

 fully washing sand or mud, collected on the beach at different levels 

 between tide-marks, or from dredgings, or by carefully search- 

 ing the surface of seaweeds, or by washing their roots, or, again, 

 by the surface or deep-sea tow-net. The sand used to weight 

 sponges for sale is the ready source of a large number of 

 forms, and may be obtained for the asking from the sponge- 

 dealers to whom it is a useless waste product. If this sand is 

 dried in an oven, and then poured into water, the empty shells, 

 tilled, with air, will float to the surface, and may be sorted by 

 fine silk or wire gauze. 



From the resemblance of the shells of many of them to the 

 iSTautilus they were at first described as minute Cephalopods, or 

 Cuttlefish, by d'Orbigny,-^ and their true nature was only elucidated 

 in the last century by the labours of Williamson, Carpenter, 

 Dujardin, and Max Schultze. At first they possess only one 

 nucleus, but in the adult stage may become plurinucleate 

 without dividing, and this is especially the case in the " micro- 

 sphaeric " states exhibited by many of those with a complex 

 shell ; the nucleus is apt to give off fragments (chromidia) which 

 lie scattered in the cytoplasm. At first, too, in all cases, the 

 shell has but a single chamber, a state that persists through 

 life in some. When the number of chambers increases, their 

 number has no relation to that of tlie nuclei, which remains 

 much smaller till brood-formation sets in. 



Tlie shell-substance, if calcareous, has one of the two types, 

 porcellanous or vitreous, that we have already mentioned, but 

 Polytrema, a form of very irregular shape, though freely 

 perforated, is of a lovely pink colour. In the calcareous shells 

 sandy particles may be intercalated, forming a transition to the 

 Arenacea. In these the cement has an organic base associated 

 with calcareous or ferruginous matter; in some, however, the 

 cement is a phosphate of iron. The porcellanous shells are often 

 deep brown by transmitted light. 



'^ The name Foraminifera was used to express tlie fact tliat the chambers 

 conimuiiicated by pores, not by a tubular siphon as in Nautiloidea and Ammonoidea 

 (Vol. III. j.p. 393, 396). 



