396 INVERTEBKATA CHAP. 



polypides are already being developed. Fixation takes place by the 

 posterior glandular pole, and then the walls of the anterior invagina- 

 tion are suddenly turned back so that the polypide area is exposed. 

 The retroverted folds adhere to the substratum and force the larva 

 away from its primary attachment. In this way a huge sucker-like 

 organ is formed at the posterior pole ; this sucker becomes a 

 completely closed sac, and then its contents are devoured by 

 amoebocytes. 



It will be seen that in the series of larvae which we have just 

 described we have to deal with a progressive disappearance of larval 

 structures, and a progressive hurrying on of adult structures. Thus 

 in the Phylactolaemata, which constitute the culminating point of the 

 series, the larval body has become merely a skin enclosing the first 

 two or three buds. It is obvious, therefore, that in seeking for 

 light on the past history of the Polyzoan stock we must confine our 

 attention to the primitive type of larva represented by Cyphonautes. 



BUDDING 



The metamorphosed Cyphonautes consists of a simple ectodermic 

 sac with a closed vesicle of columnar cells projecting into it. This 

 ectodermic vesicle is termed the polypide, and from it the ectodermic 

 parts of the tentacles, and the whole alimentary canal of the first 

 person of the colony, are derived. The mesodermal portions of the 

 tentacles, including the walls of the coelomic canals which they 

 contain, are derived from a layer of mesoderm cells (pol.mes, Fig. 

 315) which clothes the external surface of the polypide. The exact 

 origin of these mesoderm cells from pre-existing larval mesoderm 

 has not been determined. The ectodermic sac -like body of the 

 metamorphosed larva constitutes the zooecium of the first polypide. 

 The valves of the larval shell are soon shed and are replaced by 

 the continuous cuticle which constitutes the ectocyst of the zooecium. 



The first person of the colony originates therefore as a bud on 

 the body of the metamorphosed larva ; and, so far as is known, the 

 development of this bud is quite similar to that of the later buds, 

 by which the colony increases in size. It follows that in Polyzoa 

 Ectoprocta, we have not the continuous life-history of an individual 

 proceeding from the larval to the adult condition, but an alternation 

 of generations by which a sexually produced form, the larva, gives 

 rise to an asexually produced form, the first person of the colony. 



The manner in which the buds of Polyzoa Ectoprocta develop 

 has been investigated by many authors. Seeliger (1890), who 

 investigated the buds of Bugula, has given the clearest account 

 of the matter, and as his results have been confirmed in almost 

 every point by the latest observer, Eomer (1906), we shall follow 

 Seeliger in our account. 



When a new bud is about to be formed the new zooecium arises 

 as an out-pouching of the old one. The cavity of this pouch is 



