iiS FORIFERA OR SPONGES. 



small portions are sometimes set adrift from a sickly parent 

 mass. Moreover, for commercial purposes, the bath sponge 

 is cut in pieces and bedded out, the fragments reproducing 

 the whole as do cuttings from many plants. The life-history 

 of Spongilla, as told by Marshall, is one of interesting 

 vicissitudes. In autumn, the sponge begins to suffer from 

 the cold and the scarcity of food, and dies away. But 

 throughout the moribund parent, clumps of cells combine 

 into " gemmules," which are furnished with capstan-like 

 spicules, and are able to survive the winter. In April or 

 May, they float away from the parental corpse and start new 

 sponges. Some of these are short-lived males, others are 

 more stable females. The ova produced by the latter, and 

 fertilised by the cells of the former, develop into another 

 generation of sponges, which in turn die away in autumn and 

 give rise to "gemmules." The life-history thus illustrates 

 " alternation of generations." 



Development. — The development of sponges varies con- 

 siderably in the different kinds. We shall follow that of 

 a calcareous form. The ovum lying within the middle 

 stratum, but close to a canal, is fertilised by a male cell, 

 borne to it by the water. Fertilisation is followed by 

 repeated division of the ovum ; a hollow sphere of cells 

 results, and this eventually escapes from the parent into the 

 water. A short free-swimming life begins, in marked con- 

 trast to the completely sedentary state which will follow. One 

 half of the minute sphere consists of ciliated cells, the other 

 consists of larger, granular cells without cilia. The sphere is 

 soon " dimpled in " or invaginated, and the ciliated hemi- 

 sphere is surrounded by the other. A gastrula stage is thus 

 reached — a two-layered thimble-like embryo. But the cilia 

 are now inside, and the embryo fixes itself, mouth or blasto- 

 pore downwards. This very disadvantageous shut-in condi- 

 tion cannot last ; pores appear through the walls, perhaps 

 somewhat pathologically at first ; the water regains admission 

 to the internal cavity ; the cilia, which seem meanwhile to 

 have disappeared in the absence of stimulus, regain their 

 activity; an exhalent aperture (obviously in no sense the 

 mouth) is ruptured at the apex of the dome; a middle 

 stratum is derived from the inner layer, and its cells begin 

 to form spicules ; — the young sponge is made. 



