43(5 TJie Structure and Life-History of a Sponge. [Sess. 



new and more complex organism. It has been happily re- 

 marked, on this point, that " perhaps no more striking illus- 

 tration could be found of the truth of the adage that ' union 

 is strength,' than here in these two lowly, and, in their 

 isolated individuality, helpless beings, yet who, in their com- 

 bination, raise themselves in the scale of life ; . . . having no 

 stomach, yet perfect digestion and assimilation ; no heart, yet 

 free and full circulation ; no lungs or gills, yet complete 

 aeration and respiration ; no manipulatory organs, yet building 

 up a structure combining all the requisites of strength and 

 endurance, with the most wonderful lightness and flexibility."^ 

 The process of reproduction in the sponges is effected in 

 two ways — sexually and non-sexually. In winter the meso- 

 derm is usually found filled with small, yellow, oval bodies, 

 known as the winter-eggs or gemmules. These are launched 

 forth in spring into the water, when they swim about in a 

 lively manner by means of the cilia with which each gemmule 

 is provided, ultimately settling down on a rock or stone, or on 

 a piece of growing alga, when the eggs burst at the hilum or 

 pore-like opening, and the contained granules, thus scattered, 

 form a new sponge colony. This is the asexual or non-sexual 

 mode of reproduction. These winter-eggs were first noticed 

 in Spongilla fluviatilis, and were thought by Dr Johnston and 

 others to be confined to fresh-water species, but they have 

 now been found also in marine forms. In the sexual method 

 of reproduction, certain cells of the mesoderm are converted 

 into ova, and others into spermatozoa, when the impreg- 

 nated ovum undergoes segmentation in the usual manner, and 

 ultimately develops into a free-swimming embryo, before 

 becoming fixed like the winter-eggs. In this latter case, 

 however, each embryo is of course a perfect individual. Ova 

 and spermatozoa have not yet been observed in the same 

 sponge-mass, and it is hence concluded that the ova are 

 fertilised by spermatozoa which gain admission with the 

 inflowing currents of water. Some very interesting observa- 

 tions on this head are to be found in the ' Quarterly Journal 

 of Microscopical Science' for Jan. 1891, in JSTo. III. of a 

 series of papers by Mr Arthur Dendy, of Melbourne University, 

 on " Studies in the Comparative Anatomy of Sponges." Mr 

 1 'Vignettes from Invisible Life,' by John Badcock, F.K.M.S., p. 151. 



