Structure of Rhizopod Shells. 313 
pores pass much into the background, both in importance and 
development, in the presence of the principal orifice, the 
ylom ; in the Imperforata they are even entirely wanting, 
and the shell-wall can therefore be made more compact and 
solid. Further a union of the different shells in polythala- 
mous forms by means of free radial beams is unnecessary, 
but they lie with their walls directly upon each other. In 
the pylomatic siliceous shells of the Nassellaria the pores 
certainly are not inferior in their development to those of the 
perforate-concentric Spumellaria, but this is simply because 
the silica of the skeletons of itself gives them such firm- 
ness that by it a strengthening of the shell-wall and the 
consequent reduction of the pylomatic form-type is rendered 
superfluous. It is otherwise with the shells of the Chal- 
lengerida, Medusettida, and Tuscarorida, which are indeed of 
siliceous nature, although not of homogeneous consistency, 
but possess a more or less complicated internal structure, or 
consist of a mass of separate siliceous spicules cemented to 
each other. The forms belonging here therefore show dis- 
tinctly a recurrence of perforation, while the wall is at the 
same time thick. 
The character of the Spongopylida, spongy Discoidea in 
which a pylom has been formed secondarily at the margin of 
the disk, and which I have united under this character in the 
genus Spongopyle, is exceedingly instructive, and in fact 
demonstrative of the conception of these conditions here deve- 
loped. Thus Spongopyle aspera, which consists of an irre- 
gular tangle of thin siliceous rods, shows, as indicated by 
its name, a rough irregular surface ; in Spongopyle osculosa, 
S. setosa, S. craticulata, and S. Stéhrv a more uniform 
external closure is perceptible ; and this process finally attains 
its highest point in Spongopyle circularis, S. ovata, S. elliptica, 
and S. variabilis. In these forms the spongy tissue of the 
interior is shut off externally by a continuous shell, in which 
there are only some very small pores. At the margin of the 
disk is placed the pylom as a single larger orifice. By the 
development of this as the principal opening for the outflow 
of the sarcode a compact closure of the other parts of the 
spongy disk has been rendered possible, and this again, by 
the external fixing of the spongy skeletal web, and by giving 
protection against injurious external attacks, is of service. 
The phylogenetic development of an external shell-mantle 
indicated by the comparative anatomy of the species of Spon- 
gopyle is completed and confirmed by my observations upon 
the ontogeny of Spongopyle osculosa. The young stages of 
this species possess a rough surface open on all sides, and an 
