Structure of Rhizopod Shells. 313 



pores pass much into the background, both in importance and 

 development, in the presence of the principal orifice, the 

 pylom ; 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 w\ills 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 comjjlicated 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 

 w^hich 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 tliese conditions here deve- 

 loped. "J'hus Spongopyle asjyera, 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. craticulafa, and S. Siohrii a more uniform 

 external closure is perceptible ; and this process finally attains 

 its highest point in Spongopyle circularise S. ovata^ S. ellipttca, 

 and S. variahilis. 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 



