270 NATURAL HISTORY. 



on the flattened under surface of the colony, which forms an oval and contractile disc over the upper 

 side of submei'ged stones. 



The colonies, or Polyzoaria, vary in size from scai-cely perceptible objects to branched or riband-like 

 masses, in some cases several feet in length or in breadth. A moderate-sized one has been estimated 

 to contain within an area of three square inches a population of forty thousand little animals (Gosse), 

 and some of the larger growths comprise within their limits accommodation for two million 

 individual occupants. But these were not necessarily all living at one time. The colony increases at 

 the margins as fresh animals spring by budding all around, and although some of the 

 inner and the older cells may for a while be unoccupied by active living tenants, the 

 colony stili grows ; while, when one generation dies out, a second almost mysteriously 

 supplies its place, and the once empty cells are peopled with fresh occupants. For the 

 method both of individual and colonial reproduction is as varied among this diverse 

 group of animals as the nature and mode of growth of the colonial external skeleton 

 or the habits and structure of the individual, which all in turn present so many 

 divergent features as render it difficult to give a generalised account of their structure 

 and ways of life. 



Like the Corals, with which they were for so long confounded under the common 

 name of " Polypi," the Moss-animals, and with far more apparent reason, were formerly 

 regarded as vegetable organisms. Even Linnaeus, although in the end admitting the 

 wholly animal nature of the stony Corals, was never absolutely convinced that the 

 _ horny and flexible forms such as the wreath-like Corallines (Sertularia), and other 

 CKISTATELLA widely-differing animal types with which the Bryozoa were then invariably asso- 

 MUCEDO. NA- ciated. on the grounds of their common possession of an external horny or calcareous 

 (After Ai'man ) skeleton were not really members of the vegetable world. Long after, when naturalists 

 began to base their systems of classification more on the anatomical structure of animals 

 than on outward form, it became evident that the group of Corals, as then constituted, included animals 

 of very different types of structure. In 1827, Dr. Grant described the animal-inhabitant of the Sea-mats, 

 or Flustra (Saxon, jiustrian, to weave), as differing much from that of the wreath -coral lines ; and in the 

 following year M. Milne-Edwards arrived at the same conclusion. Meanwhile Dr. J. V. Thompson, 

 of Cork, had long been studying the marine productions of the Irish coast, and in 1830 he published 

 the results of his investigations on several species of plant-like animals allied to Flustra. This 

 type of animal he designated a Polyzoan, a name which at once distinguished it from the Corals. Soon 

 after, the eminent German microscopist, Ehrenberg, separated the Corals into two groups, and defined 

 several families of his class Bryozoa, in which he included, among others, the animal type previously 

 called Polyzoa by Thompson. The question as to which name should be retained for the class, long a 

 matter of debate, is still a subject of controversy. That of Thompson, undoubtedly the earlier, 

 is adopted by many British and American writers on the recent species ; while that of Ehrenberg, 

 certainly most distinctive of the class as a whole, has always been employed by Continental authors, 

 and is universally applied to the numerous fossil representatives of the class. 



The Moss-animals were next entirely withdrawn from the confines of the stony corals and radiated 

 animals ; their molluscan characters were fully recognised, and they were ranked by M. Milne-Edwards 

 with the Lamp-shells and Sea-squirts, as an inferior order of shell-fish, under the name of the 

 Molluscoida, or mollusc-like animals. With the Lamp-shells they still continue to be placed, 

 although at first sight thesa colonies of minute animals, protected by a common external structure, 

 present no obvious resemblance to or affinity with the individual Brachiopod enclosed between and 

 protected by its two-valved shell. Yet, in spite of this apparent dissimilarity, the animal inhabitant 

 of each bryozoonal cell possesses many anatomical points in common with that of the Lamp-shells, 

 so as to fully justify their joint association. 



The Bryozoa have been subdivided by Nitzsche into two principal sections, the Ectoprocta* 

 and the Endoprocta, characterised by differences in the position of the anal orifice of the alimentary 

 tube. In the more numerous Ectoprocta, the vent (anus) is situated outside the circle of the tentacles 

 surrounding the mouth (Fig. 18, B, c, a}. In the Endoprocta, it occurs close to the mouth or cral 



* Greek, tktos, outside ; endon, within ; procta, vent. 



