138 ZOOLOGY SECT, iv 



(1) these are simply mouthless hydranths with very short ten- 

 tacles abundantly supplied with nematocysts, capable of very active 

 movements, and called dactylozooids (dz). In Plumularia there 

 are small structures called " guard-polypes," resembling tentacles 

 in structure, and each enclosed in a theca. In Hydractinia the 

 coenosarc is also produced into spines (sp), which may be much 

 modified zooids. 



But the most remarkable modifications occur in the reproduc- 

 tive zooids. In a large proportion of genera, both of Anthome- 

 dusa? and LeptoinedussB, these take the form of locomotive medusa?, 

 agreeing in general structure with the descriptions already given, 

 but exhibiting endless variety in detail. As to size they vary 

 from about 1 mm. in diameter up to 400 mm. (16 inches). The 

 number of tentacles may be very great (Fig. 96, 2) or these 

 organs may be reduced to two (Fig. 96, 1), or even to one 

 (Fig. 93, 8} ; 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. 96, 3). 



Some medusa? creep over submarine surfaces, walking on the 

 tips of their peculiarly modified tentacles (Fig. 93, 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 at- 

 tachment of the velum (Fig. 89, D, nv, nv'}. The medusa? thus 

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

 systci.i, 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 medusas the circular canal communicates with the 

 exterior by minute pores placed at the summits of papilla?, 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 



