10 



The ectocyst is sometimes chitinous, at others is strengthened by 

 calcareous or silicious matter, and forms a solid wall around it, the 

 outside of which is frequently curiously sculptured. 



The colony increases by repeatedly budding. Buds arise on 

 the primary cells, which are developed into secondary cells; these 

 again divide, and so on. They are connected by what are called 

 communication plates, in which are small perforations, through 

 which minute threads pass connecting each animal with its 

 neighbour. 



The other layer of the cell, the endocyst, is more highly organised, 

 and consists of conical, nucleated cells, with their bases towards the 

 ectocyst. It is supposed by some to give rise to buds, and the 

 reproductive products, by others, to exist merely for the purpose of 

 enlarging the colony. 



This inner layer also gives origin to the muscles which enable the 

 polyzoon to carry on its various functions, by compressing the 

 membrane and lessening the size of the perivisceral cavity. 



So you see the zocecium is a chamber lined with a delicate and 

 sensitive membrane, which the polypide has built around itself, and 

 which it inhabits all its life. 



Now let us consider the anatomy of the polypides. 



They are divided into two great divisions — the Ectoprocta and 

 the Entoprocta. 



The great difference between them is that the Entoprocta have 

 both orifices of the alimentary canal within the whorl of tentacles, 

 whereas, in the Ectoprocta the anus is outside. 



There are other differences, one of which is that the Entoprocta 

 do not develop a calcareous covering ; they are much more sluggish 

 in their movements, and are furnished with contractile stems, at the 

 extremity of which the cells containing the bodies of the animals are 

 situated ; they are fixed in their cells and are incapable of protrusion 

 or retraction. 



Amongst the calcareous cells of the Ectoprocta are seen certain 

 modified cells called Oaria. They are special receptacles in which 

 the ova are developed into the larval stage. 



If we examine, carefully, a colony, we shall see that many of the 

 cells are destitute of inhabitants — the polypides have disappeared, 

 but have left behind them a brown, granular-looking substance, which 

 has been a source of contention amongst scientists, and given rise 

 to many theories as to its origin. Some maintained these brown spots 

 were merely the dead polypides, others regarded them as ova or 

 statoblasts ; now it is recognised that they are alive and in the 

 course of time develop into fresh polypides and re-inhabit the cells. 



There are many modifications in the zocecium of the marine 

 polyzoa ; not the least remarkable are the avicularia and vibracula. 



The avicularium, or the " bird's-head " process, is an organism 

 attached to the colony in the shape of a bird's head, with a formid- 

 able hooked beak, and a lower mandible worked by powerful 



