SPONGES AND ZOOPHYTES. 119 



true process of sexual generation, moreover, is said to take place in 

 Sponges; certain of the amoeboids, like certain cells of Volvox ( 240), 

 becoming s sperm-cells,' and developing spermatozoa by the metamor- 

 phosis of their nuclei; while others become * germ-cells,' developing them- 

 selves by segmentation (when fertilized) into the bodies known as 

 6 ciliated gemmules,' which are set free from the walls of the canals, 

 swim forth from the vents, and for a time move actively through the 

 water. According to Prof. Haeckel, the fertilized germ-cells are to be 

 regarded as true ova, and the products of their segmentation as morulce, 

 which, by invagination ( 391), become gastrulce; and he argues that the 

 whole system of canals and ampullaceous sacs is really, like the system of 

 canals in the Sponge-like Akyonium ( 529), an extension of the primi- 

 tive gastric cavity; the oscula of Sponges being the undeveloped repre- 

 sentatives of the polypes of the Zoophyte. As it is doubtful, however, 

 whether the supposed Sponge-spermatozoa are anything else than 

 ordinary flagellated monads, and as the development of the supposed 

 ovum by no means conforms to the ordinary gastrcea type, the question 



FIG. 352. 



3IBBK 



A. Portion of Halichondria (?) from Madagascar, with siliceous spicules projecting from the 

 keratose network. 



B. Triradiate spicules of Grantia compressa, lying in the midst of its cytoblastema. 



whether Sponges are strictly Protozoa, or are to be regarded as consti- 

 tuting the lowest formjof the Metazoic type, must be considered (in the 

 Author's opinion) as still an open one. 1 



510. The arrangement of the keratose reticulation in the Sponges 

 with which we are most familiar, may be best made out by cutting thin 

 slices of a piece of Sponge submitted to firm compression, and viewing 

 these slices, mounted upon a dark ground, with a low magnifying power, 

 under incident light. Such sections, thus illuminated, are not merely 

 striking objects; but serve to show, very characteristically, the general 

 disposition of the larger canals and of the smaller pores with which they 

 communicate. In the ordinary Sponge, the fibrous skeleton is almost 

 entirely destitute of spicules; the absence of which, in fact, is one im- 



1 See Chap. v. of Mr. Saville Kent's " Manual of the Infusoria," and Chap. v. 

 of Mr. Balfour's "Comparative Embryology," as well as Prof. Haeckel's impor- 

 tant work on the Calcareous Sponges. 



