2s8 THE BIOLOGY OF THE SEA-SHORE 



BREEDING AND GROWTH OF SOME COMMON SHORE FORMS 



Porifera. — Sponges reproduce both sexually by means of 

 ova and spermatozoa and asexually by one of three methods, 

 viz. budding, gemmulation, and formation of asexual larvae. 

 In the former case the genital products are found in all stages 

 in the middle gelatinous layer or mesogloea. According to 

 Dendy, in Halichondria panicea the eggs are fertilised in the 

 inhalant canals whither they migrate by amoeboid movements 

 and where they become suspended by means of a peduncle. 

 The free-swimming larvae : minute, oval, ciliated bodies, 

 escape by the osculum. An asexual bud is simply a portion of 

 the parent wall containing the usual three tissue layers which, 

 originating as a small projection, eventually becomes pinched 

 off, floats away, and after fixation gives rise to a new indi- 

 vidual. In the simplest sponges the bud is merely one of 

 the Ascon tubes which becomes pinched off and so liberated. 

 In Leucosoleiiia hotryoides the free, distal end of the bud, 

 instead of becoming the osculum as might be expected, 

 forms the base of the young sponge, the osculum being formed 

 where the bud is constricted from the parent. 



In Oscarella lohularis the buds are transparent floating 

 bladders, their vesicular nature being regarded as adaptive, 

 as it lessens their specific gravity and enables them to float 

 to a distance from the parent. 



Gemmulation is familiar from the well-known case of 

 Spongilla where it is a means of ensuring survival in variable 

 fresh- water conditions, gemmules, like spores, being remark- 

 able for their powers of resistance to adverse surroundings. 

 Gemmules very similar to those of Spongilla are known in a 

 few marine sponges, especially in Siiherites and in Ficulina, 



In the Hexactinellida, which are deep-sea forms and 

 consequently do not concern us here, the place of sexually 

 produced larvae is taken by bodies of similar origin to gem- 

 mules, but of structure practically identical with that of the 

 larva developed from the fertilised egg. The occurrence 

 of these asexual larvae may be associated with the difficulties 

 of life in the depths (see Sollas, 1909). 



