THE FAMILY ASTROEHEZID.E 113 



panying faunas ; for it has been shown, by Murray 

 and others, that the mud hne round continents is 

 the great feeding-ground of the various groups of 

 animal hfe, and, with equal reasoning, the starting- 

 point of many types of life, which subsequently 

 became modified by change of condition, such as 

 the abyssal for example, which, according to its 

 faunas, may be regarded as abnormal, since at the 

 present time only erratic or specialised modifications 

 of life are found there. At first thought it might seem 

 that the simplest type of test structure w^ould be 

 the arenaceous tube or sphere, but observations in 

 other groups will serve to show how a- decidedly 

 complex structure may be produced on the simplest 

 possible plan. Looking further into the case of the 

 foraminiferal shell, the essential feature of the 

 animal is its ability to send forth reticulate and 

 threadlike pseudopodia. These extruded threads 

 become, in the course of the building up or forma- 

 tion of the shell-wall, partially surrounded by shell- 

 substance along a certain line external to the main 

 body or series of segments, and thus, by a simple 

 process of calcification of the external layer, or depo- 

 sition at the bases of the pseudopods, an apparently 

 complex shell-structure may be formed, corresponding 

 with any of the hyaline types which have been found 

 in the lower paheozoic rocks. 



