Chap. III.] 



MEDUSA. 



39 



persons take on different duties; when these, again, 

 are at their simplest stage, we find that while some 

 nourish the colony (tropliosomes), they take no 

 share in reproducing it; this office is performed by 

 other persons (gonosomes), which depend for their 

 nourishment on the neighbouring tropliosomes. Di- 

 vision of labour among the persons of the colony may 

 go still farther, and groups become formed of which 

 some have nutrient, others locomotor, others protective, 

 and others prehensile or offensive functions (Siphono- 

 phora; e.g. Portuguese man-of-war) (Fig. 12). Where 

 the Ccelenterate is fixed, we observe, in one division, 

 that the generative persons become free-swimming, 

 and, while retaining the 

 essential characters of the 

 division, become greatly 

 altered in form, in adap- 

 tation to their new mode 

 of life ; such persons are 

 spoken of as Medusae 

 (Fig. 10). Finally, we 

 find that, in some cases, 

 the fertilised ovum of a 

 medusa gives rise not to 

 a fixed hydra-like body, 

 but directly to a medusa 

 form. The tentacles are 

 set round the mouth in a 

 circle, and the parts of 

 the body are similarly 

 arranged in a fashion of 

 symmetry, which is called 

 radial ; where, however, 

 the free mode of life has obtained for a long period of 

 time, we sometimes find that there is only one axis of 

 the body on either side of which exactly corresponding 

 parts are to be found ; in other words, a bilateral 



Fig. 11. Longitudinal section 

 through Sagartia parasitica, 

 showing a meseuteric septum 

 with the body wall to the right, 

 and the enteric wall to the left. 

 (After O. and E. Hertwig.) (See 

 Fig. 54.) 



