SECT, iv PHYLUM CCELENTERATA 151 



diameter up to 400 mm. (10 inches). The number of marginal 

 tentacles may be very great (Fig. 110, 2), or these organs may be 

 reduced to two (Fig. 1 10, 7), or even to one (Fig. 106, 3) ; in the 

 last-named cases it will be noticed that the medusa is no longer 

 radially but bilaterally symmetrical, i.e. it can be divided into two 

 equal and similar halves by a single plane only viz., the plane 

 passing through the one or two tentacles. With the increase in the 

 number of the tentacles a corresponding increase in that of 

 the radial canals often takes place (Fig. 110, 3). In addition to the 

 marginal tentacles longer or shorter oral tentacles may be present in 

 a whorl surrounding the mouth (Fig. 110, t'). 



Some medusae creep over submarine surfaces, walking on the 

 tips of their peculiarly modified tentacles (Fig. 106, 6), but the 

 majority propel themselves through the water in a series of jerks 

 by alternately contracting and expanding the umbrella, and so, 

 by rhythmically driving out the contained water, moving with 

 the apex foremost. In correspondence with these energetic move- 

 ments there is a great development of both muscular and 

 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 attach- 

 ment of the velum (Fig. 102, D, nv, nv'). The medusae 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 medusa 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 medusa? the circular canal communicates with the 

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

 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. 106, la], or from the margin of the umbrella (76) or the base 

 of the tentacles : in one case they are formed on blastostyles 

 developed on the gonads. 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. 106, 76, 5, s), 



