718 TRANSACTIONS OF SECTION D. 



history of these animals, particularly by his classical monograph on the Fresh- 

 water Polyzoa/ was also an Irishman, who was born at Cork, and for some years 

 held the professorship of Botany in the University of Dublin. Thomas Hincks, 

 another worker who was pre-eminent for his knowledge of the Polyzoa and for the 

 importance of his researches in this field, held professional appointments both at 

 Cork and at Dublin for several years. 



The Polyzoa are a group which is quite unknown to most persons who are not 

 Zoologists. Before coming to my special subject, the variations of the avicularia, 

 I may for this reason, perhaps, be excused for attempting to explain what the 

 Polyzoa are like, and, in particular, what are the nature and functions of the struc- 

 tures we have to discuss. 



The Polvzoa are a class of aquatic organisms of world-wide distribution and 

 includin"- a lar^e number of species. They occur both in fresh water and iu the 

 sea and'' the marine forms are found from between tide-marks to the deepest 

 abysses of the ocean. Some of the species are among the commonest objects of the 

 sea-shore, and others may be obtained in numbers by the use of the dredge or 

 trawl. They often occur as delicate encrustations, usually calcareous, on plants, 

 stones or shells ; or they may assume the appearance of sea-weeds, corals, or 

 hydroids. Although most of them are of comparatively small size, they are 

 usually large enough to be recognised by the naked eye, while the largest of them 

 reach a diameter of a foot or two. 



The Polyzoa are always colonial animals, the colony consisting of a 

 number of individuals which are in organic connection Avith one another, though 

 they may appear at first sight as a series of isolated units. Each of these units 

 consists of a body- wall, which is usually calcified and is termed the 'zocecium, 

 [since it was at one time supposed to constitute a sort of house for a zooid known 

 as the ' polypide.' The idea of a dimorphism of individuals expressed by this 

 nomenclature is no longer accepted, but the terms themselves are still conveniently 

 employed for descriptive purposes. The polypide consists iu reality of the visceral 

 mass of the zocecium, together with the series of ciliated tentacles which are used 

 for the capture of food. The tentacles are protrusible, but are commonly found 

 retracted into the interior of the zocecium, iu which condition they lie in a thin- 

 walled introvert or ' tentacle-sheath,' which opens to the exterior by an ' orifice ' 

 in the wall of the zocecium. In the sub-order Cheilostomata, to which my remarks 

 will principally refer, the orifice is closed, during the retracted condition of the 

 polypide, by a'chitinous lid or ' operculum.' 



In the oreat majority of cases the colony is inaugurated by the fixation of 

 a free-swiaming larva, which has been produced from an egg by the ordinary 

 sexual method. On the completion of its metamorphosis the larva becomes the 

 first zooecium of the colony, and is then known as the ' ancestrula,' a term intro- 

 duced by Jullieu to signify that it is the ancestor of the iuture colony. In a 

 laro-e number of species belonging to the most diverse genera of Cheilostomes 

 the^ancestrula has a certain definite character which appears to have no relation 

 to that of the individuals to which it gives rise by budding. The type of 

 ancestrula in question has a srriking resemblance to a siiigle zocecium of many of 

 the species of the existing genus Membranipora,_ and is characterised by having 

 a series of marginal spines which surround a region closed by a chitinous mem- 

 brane at one end of which is situated the operculum. That this form of ances- 

 trula 'has a definite significance is indicated by its wide occurrence among 

 Cheilostomes and by the fact that the same cannot be said of any other form of 

 ancestrula and is confirmed by the palaeontological occurrence of 3Iemb?-anipora 

 as one of the earliest genera of Cheilostomata. 



The ancestrula gives rise by budding to daughter-zoobcia, which usually 

 assume from the first the characters proper to their species. In the growing 

 colony the formation ot new zocecia takes place at the expense of a marginal zone, 

 whicli contains the tissues concerned in the bud-development. Omitting the 

 consideration ot special regenerative processes which may take place, a zocecium 



I Bay Society, 1856. 



