156 NATURE AND AFFINITIES OF THE SPONGES. 



paraded with so much tfclat, when put to the test was, to use a familiar 

 expression, found incapable of " holding water " ; it now remains to be 

 seen whether Professor Haeckel's new arguments were based upon a 

 more firm foundation. The one essential point brought forward on this 

 occasion relates to the composition and significance of those ciliated and 

 motile reproductive bodies common to all sponges, first discovered by Dr. 

 Grant, and noticed by various subsequent observers, but whose true struc- 

 ture and import have not yet been exhaustively investigated. In accord- 

 ance with Professor Haeckel's interpretations, these bodies, or " ciliated 

 larvae," as they have since been more commonly designated, abundantly 

 developed throughout the representatives of the Calcispongiae, were all 

 referable to one common plan, with regard both to external configuration 

 and internal histologic composition. This common plan, as now enun- 

 ciated, manifested itself externally in the possession of an evenly ovate 

 or subpyriform contour, the broader end representing the anterior pole, 

 as exhibited by the body in its condition of natation. Except at one 

 point, the entire peripheral surface of these bodies was clothed with long 

 vibratile cilia, each cilium originating from the centre of a minutely circum- 

 scribed polygonal area, to each of which was assigned the morphologic value 

 of a single cell. The exceptional region referred to, over which the cilia did 

 not extend, was limited to the anterior pole, from which point an axial 

 canal was described as leading from the external surface to a central body- 

 cavity. Round the outer edge of this apical opening were stationed a circu- 

 lar border of larger subspheroidal non-ciliated cells, which represented the 

 externally protruding units of a layer of similar cells that lined in a single 

 and continuous series the entire surface of the hollow internal cavity with 

 which the apical aperture was continuous. Taken thus in optical longi- 

 tudinal section, these bodies, as interpreted by Professor Haeckel, or 

 borrowing from his own illustrations, as represented at Fig. 2 in the 

 adjoining woodcut, presented the aspect of an ovate sac composed of two 

 separate, and histologically distinct, external and internal cellular layers, 

 the outer one being composed of more minute subcylindrical and radially 

 disposed monociliate cells, and the inner one of a correspondingly simple 

 layer of much larger subspheroidal and non-ciliated cells. This sac-shaped 

 bilaminated structure Professor Haeckel denominated a "gastrula," and 

 represented it to be the ground or stock-form from which all sponges 

 were primarily developed. It was further insisted that the outer or so- 

 called " dermal lamina " of this bilaminated structure represented a true 

 external dermal layer or ectoderm, and the inner or so-called "gastral 

 lamina" a true entoderm, as obtains in all ordinary Metazoic organisms. 

 Launching out into the regions of hypothesis, Professor Haeckel claimed 

 for his so-called " gastrula " a far-reaching and most important significance. 



" I regard the gastrula," Haeckel says, " as the most important and significant 

 embryonic form in the whole animal kingdom. It occurs among the sponges, the 

 Acalephae, the Annelida, Echinodermata, Arthropoda, Mollusca, and the Vertebrata 



