SPONGES 67 



overgrown gonocyte, but represent each a separate germ cell, which 

 has arisen independently of its fellows by modification of a wander- 

 ing cell. 



It is evident that were a gemmule to be composed of a single enlarged 

 statocyte, a case would arise which would be difficult to distinguish from 

 parthenogenesis. Such seems, as a matter of fact, to be the true interpreta- 

 tion of the " budding " of Tethyn, in which, according to Deszo, each bud 

 arises from one of a number of large cells, termed by him Sprosszellen 

 (germinating cells). Each Sprosszelle is contained in a capsule in the 

 cortex and gives rise by division to a multicellular reproductive body, 

 from which a small sponge develops like a bud on the surface of the 

 parent. 



Gemmules, similar apparently to those of SpoiKjillimie, have been 

 observed by Topsent in many marine sponges, not only in forms allied to 

 SpongiUa (Reniera, etc.), but also in genera so far removed from it in the 

 system as Cliona and Craniella (Tetractinellida). 



(c) Emhryologij. — All sponges, so far as is known, develop by means 

 of a ciliated larva, produced from a fertilised ovum Avhich under- 

 goes, in all cases, a total or holoblastic segmentation.^ After swim- 

 ming freely for a longer or shorter period, the larva fixes itself and 

 undergoes a complete metamorphosis, after which it develops into 

 a young sponge, with pores and osculum, which commences to feed 

 and grow. 



In Cliona, the boring sponge, the ova are extruded from the sponge 

 before segmentation has commenced, and go through their whole develop- 

 ment outside the maternal body. In all other known cases the ovum 

 goes through its early development, up to the formation of the larva, 

 within the maternal tissues. Hence the early development of sponges 

 may be divided conveniently into three periods : (1) The enih-yonic period, 

 from the ovum to the free swimming larva, usually passed within the 

 maternal tissue ; (2) the larval or free swimming period ; and (3) the 

 piqjal period, from the fixation to the formation of pores and osculum. 



There is scarcely any zoological problem which would appear, from a 

 study of the literature alone, to be so confused and difficult as the 

 embryonic development of sponges. The difficidty proves, however, to 

 be due not so much to the nature of the objects themselves as to the 

 many prejudices and preconceived notions with which they have been 

 studied. We may commence the account of this chapter in sponge 

 morphology with the life-history of a very simple and typical form, such 

 as Clathrina hlanca, in which the adult structure is in all respects similar 

 to that of the Olynthus already described. The embryology of the 

 remaining types may then be studied from a general point of view, by 

 comparing, first, the various types of larva, and secondly, their meta- 

 morphosis and organogeny. 



' For fertilisation see above, p. 61. 



