490 



NA TURE 



[Skptemcer 17, 1908 



In the fifth memoir, published at Cork in 1830, of a short 

 but brilliant series of Papers,' Thompson was the first to 

 demonstrate the essential nature of the differences between 

 the Polvzoa and the other " Zoophytes " with which they 

 had previously been classified. G. J. Allman, who at a 

 later period did so much to throw light on the structure 

 and natural 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 Polvzoa 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 structures we 

 have to discuss. 



The Polyzoa are a Class of aquatic organisms of world- 

 wide distribution, and including a large number of species. 

 They occur both in fresh water and in 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 with 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 "zooecium," since it was at one time 

 supposed to constitute a sort of house for a zooid known 

 as the " polypidc." The idea of a dimorphism of in- 

 dividuals expressed by this nomenclature is no longer 

 accepted, but the terms themselves are still conveniently 

 employed for descriptive purposes. The poh'pide consists 

 in reality of the visceral mass of the zooecium, 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, 

 in 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 zooecium. In the suborder 

 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 great majority of cases the colony is inaugurated 

 by the fixation of a free-swimming larva, which has been 

 produced froin 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 introduced by Jullien to signify that 

 it is the ancestor of the future colony. In a large number 

 of species belonging to the most diverse genera of Cheilo- 

 stomes 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 striking resemblance to a single zooecium 

 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 membrane, 

 at one end of which is situated the operculum. That this 

 form of ancestrula 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 ances- 

 trula, and is confirmed by the palieontological occurrence 

 of Membranipora as one of the earliest genera of Cheilo- 

 stomata. 



1 " Zoological Researches and Illustrations." 

 - Ray Society, 1856. 



NO. 2029, VOL. 78] 



The ancestrula gives rise by budding to daughter-zocecia, 

 which usually assume from the first the characters proper 

 to their species. In the growing colony the formation of 

 new zooBCia takes place at the e.-cpense of a marginal zone, 

 which contains the tissues concerned in the bud-develop- 

 ment. Omitting the consideration of special regenerative 

 processes which may take place, a zoujcium which has 

 once been constituted at the growing margin of the colony 

 does not, as a rule, possess the power of giving rise to new 

 zocBcia, although it commonly has the faculty of producing 

 sexual cells from which free larvas may develop. 



In the majority of the species of Cheilostomata many 

 of the individuals of the colony have the forin of the so- 

 called avicularia. \n avicularium is characterised by 

 possessing a chitinous " mandible," which can be closed 

 with great force by strong occlusor muscles, the organ 

 being thus essentially of a prehensile nature. There can 

 be little doubt that the mandible is a modification of the 

 chitinous operculum which closes the orifice of the tentacle- 

 sheath in Cheilostomata. It thus follows that avicularia 

 are restricted to this particular division ' of the Polyzoa. 

 In the avicularium the operculum has become relatively 

 and often absolutely enlarged, and its muscles have be- 

 come more powerful than those of the unmodified zooecia. 

 The internal viscera have, as a rule, disappeared, and there 

 are thus neither tentacles nor alimentary canal. The 

 body-wall, or zooecium, has become a case which contains 

 the muscles, while part of it has been prolonged into a 

 beak-like structure or " rostrum," which, with the chitinous 

 mandible, constitutes the prehensile mechanism. 



In Bugula and its allies the avicularium has the form 

 to which its name refers, and has a striking resemblance 

 to the head of a bird like an eagle or vulture. This 

 resemblance is due, not only to the general form of the 

 structure, but also to the hooked and beak-like shape of 

 its rostrum and to the narrow neck by which it; is con- 

 nected with the zocEcium on which it is borne. The 

 avicularia of Bugula have considerable powers of move- 

 ment, and in the living condition they may be seen to 

 bend backwards and forwards on their flexible neck, their 

 range of action being thus considerably enlarged. The 

 mandible is ordinarily held wide open, but it closes with 

 great force when some foreign object is placed between 

 the jaws. An avicularium which has in this way seized 

 a small worm, for instance, is known to be able to retain 

 its capture for many hours, in some cases for more than 

 an entire day. 



In the majority of Cheilostomata the avicularia are, 

 however, not stalked. They occur scattered over the colony 

 in a considerable variety of positions, and usually appear 

 as appendages rigidly connected with the walls of the 

 zooecia. 



More than one attempt has been made to explain the 

 functions of the avicularia. The distribution of these 

 organs indicates, I think, that the simplest and most 

 obvious explanation is the correct one — namely, that, like 

 the pedicellariaa of Echinoderms, they are defensive organs. 

 The ordinary uninodified opercula probably have the same 

 function in many cases ; and if account be taken of the fact 

 that the avicularium is morphologlcallv a modified zoceciimi 

 it becomes easy to understand that the defensive office of 

 the opercula has been made more efficient in specially 

 modified zocecia which concentrate their energies on this 

 one function alone. 



K casual inspection of a number of Cheilostomes taken 

 almost at random reveals the fact that the avicularia are 

 specially common in the immediate neighbourhood of the 

 orifice of the tentacle-sheath and of that of the " com- 

 pensation-sac." 



This is a thin-walled cavity which in a considerable 

 proportion of the Cheilostomes opens to the exterior at 

 the proximal border of the operculum. Its principal func- 

 tion is to permit protrusion and retraction of the polypide 

 to take place, since in a zooecium with completely rigid 

 walls the act of protrusion could not occur if the temporary 

 removal of structures of considerable size were not com- 

 pensated for by the admission of water into the space 

 included by the rigid body-wall. At each movement of 



1 Tt may be noted that Palreontolog'sts have descrihed structures which 

 they have regarded as avicularia in Polyzoa which do not belong to the 

 Cheilostomata. 



