ON SHALLOW WATER FAUNAS. 85 



adult is attached, and these include a large proportion of the 

 shallow-water fauna, the animal in the early stages of its 

 existence leads a free swimming life, and is often entirely 

 unlike the parent in appearance and structure. This fact, 

 which will be referred to again further on, is to be regarded 

 as evidence of the strongest kind that the attached condition 

 is a secondary one, acquired in accordance with the habitat 

 of the animal ; in other words, that the attached forms are 

 descended from free-swimming ancestors. 



A large number of these attached forms, especially in 

 the more lowly organised groups, e.g., Sponges and Coelen- 

 terates, have the power of reproducing asexually, as well as 

 by means of eggs, the asexual process being usually one of 

 budding. A bud is commonly a hollow process of the body 

 wall of the parent, w^hich gradually increases in size and 

 complexity until it becomes a second animal in all respects 

 like its parent. It may then either separate and become a 

 distinct animal, or, as is more usually the case, may remain 

 permanently attached to, and in communication with the 

 parent. Both the bud and the parent may give rise to 

 fresh buds, and in this way a "colony" is formed, this 

 being the name given to such an aggregation of individuals 

 formed by budding, and remaining organically connected 

 together. 



Such colonies are not confined to the shallow-water 

 fauna, but are far more abundant and more varied there than 

 in either the deep sea or the open ocean, and hence may be 

 very suitably considered in this place. 



From their mode of formation, described above, it 

 follows that the several members of a colony are funda- 

 mentally equivalent to one another. In the sim-pler forms 

 they remain all alike, and, although organically connected 

 with one another, still practically independent. Examples 

 of such colonies are presented by many of the Polyzoa, 



