66 COELENTERATA [CH. 



its parent, the Medusa, was separated. The free-swimming young 

 or planulae furnish good examples of what is meant by the term 

 larva. This name is given to the young form of any animal when 

 it is very different to the fully-grown animal and leads a free life. 



We have thus learnt that a Medusa gives rise to an egg which 

 develops into a Hydroid person, which after a time in turn buds off a 

 Medusa; such an alternation of generations is very characteristic 

 of a large number of Coelenterata. The Medusa repre- 

 O f Generations sents a sexual generation, the Hydroid an asexua.l 

 generation, and inasmuch as the Medusoid is often 

 only produced as a bud of the third or fourth order (i.e. is budded 

 from a hydroid person which was produced similarly from another 

 Hydroid person), it will be seen that several asexual generations 

 intervene between two sexual ones. One explanation of this life- 

 history is that the Medusa is only a specially modified Hydroid, which 

 has acquired the power of locomotion in order to disperse the eggs 

 over a large area, and thus avoid the overcrowding of a limited area 

 with one species. The swimming bell and velum are contrivances 

 to enable the bud which bears the eggs to move about. If, however, 

 this explanation be adopted, it is a most remarkable fact that in 

 many species the Medusae are very imperfectly developed and 

 never become free. Such Medusae are usually more or less de- 

 generate and are termed gonophores. Since the gonophore fails 

 to fulfil the purpose for which we believe the Medusa to have been 

 developed we must assume that conditions have so far changed 

 that the same wide scattering of the eggs is not now so necessary as 

 formerly, possibly because the species in question are restricted to . 

 particular strips of the shore. It is an interesting fact that those 

 species in which the hydroid persons develop strongly and bud 

 frequently, so as to form a complicated branching system, generally 

 have degenerate Medusae, whilst in those species on the contrary 

 which have free Medusae the hydroid stock buds feebly or not at all 

 and is usually small and poorly developed. Tubularia larynx found 

 growing on seaweed is a good example of a form with degenerate 

 Medusae, Bougainvillia or Obelia of forms with free Medusae. 



The Hydromedusae include a large number of families, most of 

 which are represented by small plant-like forms resembling the 

 genera just mentioned, but there are several groups which show 

 marked peculiarities and have been regarded by many zoologists as 

 of co-equal rank with the order although they have doubtless 

 been derived from ordinary Hydromedusae. Of these we may name 



