II _ HYDROZOA 101 



medusae — budded off in this case by the ordinary polyps round the base 

 of the oral cone (Fig. 40, M) — never become completely free but remain 

 attached to the parent^ looking eventually like minute bunches of grapes 

 surrounding and sometimes hiding the oral cone almost completely. 

 But though they never becpme free their structure is still clearly 

 anthomedusan — they develop gonad on the surface of the manubrium; 

 and the young hydroid individuals arising from the zygotes go on with 

 their development within the shelter afforded by the umbrella. 



Tubularia and Obelia illustrate two main types of structure and 

 life -history found within the group Hydromedusae which is conse- 

 quently divided into two sub-groups namedj accordingly as the Hydroid 

 or the Medusoid structure is regarded as more important : 



(i) Gymnoblastea or Anthomedusae 

 exemplified by Tubularia 

 and (2) Calyptoblastea or Leptomedusae 

 exemplified by Obelia. 



The group contains a great variety of different genera and species, 

 including many of our commonest marine animals, and constituting a 

 large proportion of what were known to the older naturalists as Zoophytes. 



ACALEPHAE 



The third subdivision of the Hydrozoa, the Acalephae, includes the 

 ordinary large jelly-fish or medusae such as are commonly seen swimming 

 in the sea or cast up on the shore. One of the commonest of them, 

 Aurelia — easily distinguished by the bright purple colour of the gonad 

 which is in the form of four conspicuous ring-shaped or horseshoe-shaped 

 masses — may be taken as an example of the group. 



The general shape is similar to that of a Leptomedusa but there are 

 characteristic differences in detail. The size is much greater. TheJQur 

 angles_of_th^_j]aouth are drawn out into long frilled structures. The 

 radial canals (Fig. 42, r.c) are sixteen in number and those opposite 

 the angles of the mouth (" per-radial ") as well as those exactly midway 

 between them (inter-radial) branch as they pass outwards, while the eight 

 alternating with these (adradial) retain their simple unbranched character. 

 The stomach bulges outwards in the form of four rounded pockets, 

 inter-radial in position, and the ovaries or testes are in the form of four 

 horseshoe -shaped thickenings of the endoderm of the floor of these 

 pockets (Fig. 44, 1,^). Probably for the purpose of bringing the sea-water 



