PORIFERA 



65 



points of the body surface. These, without detaching themselves, 

 grow larger, and can in their turn form buds. Sponge colonies thus 

 arise. As buds may at various points groiv together, the colony itself 

 may again have the appearance of a plexus or framework. The holes 

 and interspaces of such a colony may then again assume the character 

 of a canal system (pseudo-canals). These must, however, according 

 to their origin, be sharply distinguished from the real canal system 

 which runs through the walls of every sponge individual Separate 

 sponge indi^dduals may also fuse and form colonies. The number of 

 oscula generally corresponds with the number of individuals which 

 form the colony (Fig. 56, o). 



In the so-called internal gemmation groups of cells called 

 gemmulse detach themselves from the sponge body, and after a 

 period of rest develop into complete sponges. Observers differ as to 

 the finer processes which take place during the development of these 

 gemmulfe. 



Sexual reproduetion. — Sponges are either hermaphrodite or 

 disecious ; in the former case the eggs and spermatozoa are not pro- 

 duced at the same time in the same individual or colony ; they are 

 protandrously hermaphrodite. The eggs and spermatozoa seem to 

 develop from mesoderm cells. 



Development. — The course of development of the sponge from the fertilised egg, 

 ivhioh often begins within the mother body, seems, to judge from the as yet insuifi- 

 cient and often contradictory observations which have been made, to be so varied 



Fio. 61.— Sections of three stages of development of Oscarella lobularis, after K. Helder. 

 A, Gastrula which has attached itself. B, Kudiments of mesoderm and canal system. C, Forma- 

 tion of the osculum and ciliated chambers, e. Ectoderm ; &n, endoderm ; ?», mesoderm ; o, osculum ; 

 ■p, pores ; wk, ciliated chambers. 



that it is hardly possible to form a generally applicable scheme. AVe select the 

 newly investigated development of Oscarella {Halisarca) lobularis (Fig. 61). 



By means of repeated egg division a freely swimming larva arises, the Blastula. 



This is a hollow sphere, whose wall consists of one single layer of flagellate cells. 



The blastula by invagination becomes a gastrula. This attaches itself by the 



gastrula mouth or blastopore, and the aperture gradually narrows and finally closes. 



VOL. I F 



