88 LIFE IN THE SEA [PART I 



formation of buds from this original zoid. So also with the 

 zoophytes. These are plant-like organisms belonging to the 

 coelenterata, and each colony is an organism which consists of a 

 great number of separate units or polyps (or zoids). Here again 

 we begin with one original individual which has budded to 

 produce the colony. Corals are also colonial animals, and in the 

 massive coral formations we see numbers of separate stony 

 receptacles each of which is the house of a separate coral zoid. 

 All of these are formed by the budding of one or more zoids. 

 Siphonophores are also compound animals in which the zoids are 

 formed by budding, and in these animals the division of labour 

 among the zoids, which is also exhibited by some other compound 

 animals, is well shewn. In a siphonophore there are assimilatory, 

 sexual, locomotory and defensive zoids, all of them produced by 

 the modification of originally similar individuals. In all compound 

 animals the zoids are not entirely free from each other but all are 

 connected together by a common flesh and are set upon a common 

 skeleton. Each zoid may sometimes exercise the sexual function. 

 Eggs and spermatozoa may thus be produced by each if they are 

 hermaphrodite, or separate zoids may form these elements. Each 

 colony begins in this way. 



Then we have the method of reproduction by simple division 

 which is generally characteristic of the protozoa, but is also 

 exhibited by groups of animals much higher in the scale. In this 

 mode the organism simply divides into two. But this process 

 cannot continue indefinitely, and a phase in the life of each 

 individual is attained in which reproduction by the union with 

 another takes place. Here there is an alternation of generation. 

 Sexually produced and asexually produced series recur with more 

 or less regularity. Alternation of generations is the rule in the 

 vegetable kingdom and is characteristic of several animal groups. 

 Even in the higher sub-kingdoms of the animals there are evi- 

 dences of an alternation of generations, though these are greatly 

 masked. Thus even in the directly developed skate there are 

 indications of the appearance of a larval stage in the embryology 

 of the beast. 



We may speak of the life-history of the majority of the animals 

 in the sea as exhibiting various stages. There is (1) the embryonic 



