PHYLUM CCELENTERATA 



151 



nervous systems. The velum and the sub-umbrella possess 

 abundance of muscle-fibres, presenting a transverse striation, 

 and round the margin of the umbrella is a double ring of nerve- 

 cells and fibres, one ring being above, the other below the at- 

 tachment of the velum (Fig. 101, D, nv, nv'). The medusas thus 

 furnish the first instance we have met with of a central nervous 

 system, i.e. a concentration of nervous tissue over a limited area 

 serving to control the movements of the whole organism. It has 

 been proved experimentally that the medusas is paralysed by 

 removal of the nerve-ring. Over the whole sub-umbrella is a 

 loose network of nerve-cells and fibres connected with the nerve- 

 ring, and forming a peripheral nervous system. 



In some medusas the circular canal communicates with the 

 exterior by minute pores placed at the summits of papillas, the 



FIG. 110. Diagram illustrating the formation of a sporosac by the degradation of a medusa. A, 

 medusa enclosed in ectodermal envelope (fs) ; B, intermediate condition with vestiges of 

 umbrella () and radial canals (/-a) ; C, sporosac. ec. ectoderm ; en, endoderm ; m, manubrium ; 

 ov, ovary ; t, tentacle ; r, velum. (From Lang's Comparative Anatowi.) 



endoderm cells of which contain brown granules. There seems to 

 be little doubt that these are organs of excretion, the cells with- 

 drawing nitrogenous waste-matters from the tissues and passing 

 them out through the pores. If we except the contractile 

 vacuoles of Protozoa, this is the first appearance of specialised 

 excretory organs in the ascending series of animals. 



Besides producing gonads, some medusae multiply asexually by 

 budding, the buds being developed either from the manubrium 

 (Fig. 105, 7a), or from the margin of the umbrella (7b) or the base 

 of the tentacles. The buds always have the medusa form. 



In many Leptolinae the reproductive zooids undergo a degrada- 

 tion of structure, various stages of the process being found in 

 different species. Almost every gradation is found, from perfect 

 medusae to ovoid pouch-like bodies called sporosacs (Fig. 105, Ib, 

 5, s), each consisting of little more than a gonad, but showing an in- 

 dication of its true nature in a prolongation of the digestive cavity 



