422 



ANIMAL LIFE-HISTORIES 



possession of gill-slits. This is a clear case of Degeneration, and 



is correlated with the fixed habit of life. 



Many of the fixed Tunicates are colonial, as the result of a 



process of budding, though they also produce eggs from which 



individuals are developed 

 that found fresh colonies. 

 Vegetative propagation is 

 unknown among verte- 

 brates, except in this 

 lowly group, and in cer- 

 tain relatives of the 

 Acorn-headed Worms. 



The floating popula- 

 tion of the sea includes 

 a number of transparent 

 Tunicates. Among these 

 are the Salps, which illus- 

 trate the kind of alterna- 

 tion of generations (meta- 

 genesis) elsewhere de- 

 scribed for the hydroid 

 zoophytes (see p. 349). 

 In these creatures there 

 are two stages in the life- 

 cycle — (i) an egg-produc- 

 ing stage, and (2) a bud- 

 ding stage (fig. 948). The 

 latter is known as a Soli- 

 tary Salp, from the hinder 

 end of which a lone stringr 



of egg-producing forms grows out, these being called Chain Salps, 



because they are at first united together. Later on the chains 



break up, and the eggs of each individual develop into the solitary 



stage. 



LIFE-HISTORIES, &c., OF FISHES (Pisces) 



Under this heading will be considered some of the more 

 interesting facts regarding Round-Mouths (Cyclostomata), Sharks 

 and Rays (Elasmobranchii), and Ordinary Bony Fishes (Tele- 

 ostei). 



Fig. 948. — A Salp (upper side), enlarged 



a, Egg-producing stage; b, budding stage; c, hinder end of ^ 

 {further enlarged); aji, atrial opening at hinder end out of which 

 water streams ; end, endostyle (a groove on floor of pharynx); ej?ib, 

 embryo of budding-stage; h, heart; k, gill; in, mouth; uia, protec- 

 tive covering; jjib, muscle-band; «, central nervous system; ?iu, 

 stomach and intestine; si, chain of egg-producing individuals (bud- 

 ding from b); wg, sense-organ. 



