SURVEY OF CCELENTERATES. 123 



rich in stinging-cells ; they are perhaps the food-killers, 

 defenders, and the sensitive critics of the colony. Lastly, 

 the hard-skinned connecting roots are raised every here and 

 there into little spines, which according to some naturalists 

 are abortive " persons," like the thorns or abortive branches 

 on the hawthorn hedge. When danger threatens, the other 

 members of the colony cower down (for like the Hydra they 

 are very contractile), and the spines are left projecting. So 

 besides budding, another fact must be kept in mind if we 

 are to understand Coelenterates, namely, the tendency 

 towards division of labour and difference of structure among 

 the members of a colony. 



The next step is more difficult to understand : it has to do 

 with the connection between many a zoophyte and what are 

 called swimming-bells or medusoids. In Hydra, individuals 

 are budded oif, but they are quite like the parent, and when 

 they are set adrift they move very little. But in many zoo- 

 phytes (Tubularians and Campanularians), among the ordinary 

 buds which add to the extent and gracefulness ■of the 

 colony, other buds appear which are after a time quite differ- 

 ent, and are detached as locomotor swimming-bells. Now 

 these bells or medusoids are sexual, they produce male and 

 female elements, they give rise to embryos, each of which, 

 after a brief life of freedom, settles down and starts a 

 zoophyte colony. There is therefore an alternation in the 

 life-history of such forms ; a fixed, plant-like, asexual 

 hydroid buds off locomotor, active, sexual medusoids, whose 

 embryos start zoophytes afresh. This alternation of genera- ' 

 tions is another complication which must be borne in mind 

 in the study of Coelenterates. 



But there are many medusoid organisms which are quite 

 like the liberated locomotor buds of Tubularians and Cam- 

 panularians, yet they seem to have no connection with any 

 hydroids, and their embryos grow into forms like themselves. 

 They are medusoids which are entirely free. Moreover, just 

 as there are colonies of polypes, so there are colonies of 

 medusoids, like the Portuguese-men-of-war, colonies in which 

 the division of labour is not less beautiful than it is in 

 Hydractinia. 



From the swimming-bells or medusoids that we have just 

 spoken about, it does not seem a great step to the jelly- 



