SPONGES 857 



its interior ; and the amceboids that form the wall of this cavity 

 Income metamorphosed into collared flagellate cells whose flagella 

 project into it. Thus is formed one of the characteristic ' ampul- 

 laceous sacs.' which, at first closed, afterwards communicates with 

 the exterior, on the one hand by an incnrrent passage, and on the 

 other with the excnrrent canal-system leading to the oscula. ]!< 

 sides this reproduction by microspores,' there' is another form 

 of non-sexual reproduction by macrospores. which are clusters of 

 amceboids encysted in firm capsules, frequently strengthened on 

 their exterior by a layer of spicules of very peculiar form. These 

 'seed-like bodies.' which answer to the encysted states of mam 

 protophytes. are met with in the substance of the sponge, chiefly in 

 winter; and after being set free through the oscula they give exit 

 to their contained am<rboids. each of which may found a new colony. 

 A true process of sexual generation, moreover, is known to take place 

 in sponges, certain of the amceboids, like certain cells of 1'<>tcti.>-. 

 becoming 'sperm-cells.' and developing spermatozoa by the meta- 

 morphosis of their nuclei: while others become -germ-cells.' 

 developing themselves by segmentation (when fertilised) into the 

 bodies known as '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. In a word, there is true sexual repro- 

 duction by ova and spermatozoa, as in all animals that are not 

 Protozoa . l 



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 view- 

 ing 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 important condition of that flexibility and 

 compressibility on which its uses depend. When spicules exist in 

 connection with such a skeleton, they are usually either altogether 



*/ 



imbedded in the fibres, or are implanted into them at their base- ; 

 but smaller and simpler sponges, such as (,'r<tntia. have no horny 

 skeleton, and their calcareous spicules are imbedded in the general 

 substance of the body. Sponge-spicules are much more fre 

 oiiently silicious than calcareous ; and the variety of forms pre- 

 sented by the silicious spicules is much greater than that which 

 we find in the comparatively small division in which they are 

 composed of carbonate of lime. The long needle-like spicules. which 

 are extremely abundant in several sponges, lying close together 

 in bundles, are sometimes straight, sometimes slightly curved ; 

 they are sometimes pointed at both ends, sometimes at one only; 

 one or both ends may be furnished with a head like that of a 



1 See Chapter V. of ilv. Saville Kent's Man mil nfthf Iitfi<nt-iu, and Chapter V. of 

 vol. i. of Mr. Balfour's Comparative Embryology, as well as Professor Haeckel's im- 

 portant work on the Calcareous Sponuc--. 



