NATURE AND AFFINITIES OF THE SPONGES. 



157 



as represented by Amphioxus. In all these representatives of the most various 

 animal stocks, the gastrula preserves exactly the same structure. From this 

 identity of the gastrula in representatives of the most various animal stocks, from the 

 sponges to the Vertebrata, I deduce, in accordance with the biogenetic fundamental 

 law, a common descent of the animal phyla from a single unknown stock-form, 

 gastraea or archegastrula, which was constructed essentially like the gastrula." 



Speculating still further with reference to this newly-detected keystone, 

 Haeckel maintained that the primary stock-form of all the sponges must 

 have been an attached, bilaminate, gastrula-like organism, in all ways com- 



FIG. i. 



FIG. 2. 



Hypothetical Ciliated Larva or so-called " Gastrula " of a Calcareous Sponge, Levcvlmis echinus 

 Hkl., as viewed, Fig. i, superficially, and Fig. 2 in longitudinal section. In Fig. 2 an imaginary central 

 gastric cavity, having a terminal but non-existing oral aperture and imaginary lining of subspheroidal 

 eudodermal cells, is represented. In Fig. i the most anteriorly developed cells of this imaginary endodermic 

 layer are projecting around the hypothetic oral aperture. After Haeckel, Monograph of the Calcispongue, 

 pi. xxx. figs. 8 and 9, 1872. 



parable with this typical " archegastrula," possessing a simple, double- 

 walled, sac-shaped body, with a central stomachal cavity, terminal oral 

 aperture, and no porous system. Should the walls be strengthened with 

 spicula, such a type might be designated Prosycum, or, in its still more 

 simplified form without any spicula, ArcJiespongia. Such a typical hypo- 

 thetic A rchespongia, though not actually known to exist, Haeckel anticipated 

 to probably possess a close ally in the singular form first described by Dr. 

 Bowerbank under the title of Haliphysema Tumanowiczii. Haeckel's later 

 and most ingenious exploits with this self-selected type will receive due 

 notice presently. 



Summing up the deductions and speculations concerning the affinities 

 and significance of the so-called sponge-larva or gastrula, just enumerated, 

 it is almost impossible to overrate the important and far-reaching issues 

 involved in Professor Haeckel's interpretation of this developmental struc- 

 ture. Presuming his interpretation to have been substantiated, and to have 



