Development of the Calcispongise. 53 



does not originate by cleaving of the mesoderm, and may even 

 exist without it ; thus, a body-cavity exists in several larvae 

 of Coelenterata in the space between the ectoderm and ento- 

 derm ; 2, the Sponges possess the mesoderm in the form of 

 the skeleton-forming layer (see above) ; 3, the inner cavity of 

 the Echinodermata (which Hackel regards as a true " body- 

 cavity ") is enclosed by the entoderm, as it is produced as 

 a derivative of the primitive intestine. Thus we see that 

 Hiickel's three fundamental opinions will not hold good ; and 

 for this reason the fourth point remains without a foundation. 

 The whole question of Coelenterism turns upon the idea of 

 the body-cavity. As soon as we without further consideration 

 conceive the inner cavity of the Vermes, Echinodermata, &c. 

 as a " true body-cavity," we place ourselves upon false ground ; 

 for that which in different animals acts as a body-cavity, re- 

 presents structures which are morphologically quite different. 

 Thus we see that in the Echinoderm-larvEe a spacious body- 

 cavity is formed which stands in no genetic connexion with 

 the definitive cavity of the body ; the latter originates in the 

 interior of the so-called lateral disks, which, in the last resort, 

 take their origin from the primitive intestine. The Ccelenteric 

 apparatus is to be paralleled with the peritoneal cavity of the 

 definitive Echinoderm-body, not witli the body-cavity of the 

 Echinoderm-larva. 



EXPLANATION OF PLATE II. 



Fig. 1. Portion of a transverse section through the Si/can-tube with two 

 segmented germs. 



Fig. 2. A segmented germ with segmentation-cavity (e). 



Fig. 3. A somewhat later stage. 



Fig. 4. Portion of a transverse section with an embryo. 



Fig. 5. The free-swimming larva : g, aggregation of granules. 



Fig. 6. A later larval stage: d, individual cells; g, aggregation of granules. 



Fig. 7. Afree-swimminglarva with skeleton-formation already commenced. 



Fig. 8. A somewhat further developed larva : d, individual cells. 



Fig. 9. An attached larva without calcareous skeleton . o, orifice of in- 

 vagination (half diagrammatic) ; d, individual 'cells. 



Fig. 10. An attached larva with calcareous spicules : a, outer ; b, inner 

 layer. 



Fig. 11. A young Sgcon, three days old. 



Fig. 12. A similar stage, treated with acetic acid : a, b, as in fig. 10 ; 

 c, gastrovascular cavity. 



Fig. 13. A young sponge, six days old. 



Fig. 14. The same treated with acetic acid : a, b, c, as in figs. 10 & 12. 



Fig. 15. A free-swimming larva of Reniera from the Crimea : e, outer 

 ciliary layer ; m, inner skeleton-forming cell-mass. 



