82 



HARDWICKE'S SCIENCE-GOSSIP. 



beautifully covered with calcareous coatings than 

 others, either for the purposes of study, or to charm 

 other eyes with its delicate beauty, as curious as their 

 own. 



If there be any desire on the part of the reader to 

 learn the history, or to unravel tlie characters of the 

 two common seaside objects, he' will be amply re- 

 compensed if he takes up any book describing the 

 Polyzoa or the Hydrozoa, to be found even on our 

 own British coast. The incrustations common on 

 shells, stones, and weeds, on every shore, belong to 

 what we prefer to call Polyzoa;* the "sea mats" 

 are included with these, and the waving plumes, 

 Sertularia and Plumularia, belong to the Hydrozoa. 

 In this series of articles, I shall direct the attention 

 of the reader to the Polyzoa, the first of these two 

 groups. 



All the marine Polyzoa belong to three very im- 

 portant suborders ; and one other group, frequently 

 found in rivers and pools, belongs to what are called 

 the freshwater Polyzoa, so admirably worked out 

 and classified by Professor Allman. Only two out 

 of the four groups are found as fossils ; one group, in 

 some one or other of these forms, ranges in time from 

 the lowest Silurian beds to the present seas ; the 

 other having genera, or species, dating back to the 

 Cretaceous era, and some of these not earlier than 

 Eocene times. It will serve our purpose, if we take 

 two types from the remaining suborders, and allow 

 these to be the representatives of each, one of wliich 

 is called the Cheilostomata, and the other the Cyclo- 

 stomata. 



In the pools left by the receding tide, and some- 

 times cast upon the shore, we often meet with large 

 fronds of Lamittaria digitata, literally covered with 

 the beautiful Polyzoa Membrattipora membranacea. 

 There is scarcely any limit to the rapid development 

 of this species, and Dr. Landsborough mentions a 

 specimen on Laminaria, five feet in length by eight 

 inches in breadth. This is large, but it is a common 

 thing to obtain specimens two feet long by two or 

 three inches in breadth. A common liand-glass, 

 magnifying from five to fifteen diameters, will show 

 the structure very well ; but it requires a magnification 

 of from 50 to 200 diameters, to reveal much of the 

 hidden beauty, and a much higher power before we 

 can say we know the special details to which we wish 

 to direct the attention of the reader. By applying 

 the hand-glass to specimens it will be seen that the 

 fronds of Laminaria are covered with silvery cells, 

 of an oblong character, disposed in lines. The cells 

 are alternate, and at each angle there is a stout hollow 

 spine. The whole of the front of the cell between 

 the boundary walls is called the area, and this is 

 covered with a delicate membrane, except at the 

 uppermost part, where there is a semicircular or 

 crescentic opening. This is the orifice or mouth. 



• They are called Bryozoa by our continental friends. 



Under this membranous covering, the polypide, an 

 animal with about twenty very long tentacles, may 

 be found in a living state in some of the cells, if the 

 Laminaria and the Polyzoa are taken and preserved 

 under favourable conditions. We must not suppose 

 that the animal lived in the whole of the space below 

 this membranous covering. The common morphology 

 of a species of Polyzoa, and Meinbranipora mem- 

 branacea is no exception to the rule, may be seen in 

 reference to fig. 55. Here we find that a common 

 cell, either membranous or calcareous, has two 

 distinct parts, an outer cell (the home of the polypide) 

 and the polypide. The polypide is a kind of tube, 

 sometimes called an alimentary zooid, bent upon 

 itself, having two orifices — the mouth and the anal. 

 This tube completely closes up the cell during life, 

 and the vacant spaces which the tube does not 

 occupy are filled with fluid. The cell is independent 

 of the animal, but it would not be exact science to 

 say the polypide is independent of the cell, though 

 in one sense such is the fact. 



Supposing then that the reader desires to know 

 something of the Polyzoa generally, we will take 

 portions of Memhi-anipora membranacea and subject 

 them to an analysis. "When the Laminaria and Polyzoa 

 are washed in fresh water, to get rid of the excess of 

 salt, it is ready for operating upon. The best way is 

 to mount or examine several large fragments, so as to 

 get a general knowledge of its structure. There are 

 an immense number of cells closely packed together — 

 every one apparently separated one from the other — 

 yet every one is connected. All the closely packed 

 cells would be called the polypidom or polypary by 

 Johnson and others ; it is now known by the term 

 ccenecium, or in other words the common dermal or 

 skin system of a colony. If we take a portion of the 

 colony and scrape it on to a glass, add a little turpen- 

 tine or benzole and then mount in Canada balsam, 

 we shall have transparent sections of several cells in 

 all manner of positions ; or we can make sections of the 

 Laminaria andMembranipora, and prepare and mount 

 these so as to show a side view of both. Using toler- 

 ably high powers ranging from J to 1 inch objectives, 

 we shall be able to detect perforations in the side and 

 end walls of the cells, composed of much thinner 

 material than the ordinary substance of the cell walls. 

 There are several of these in the species under de- 

 scription. These are " Rossetten Platten" (fig. 55). 

 of Reichert, or " communication pores " of Smitt and 

 Hincks. In all the Cheilostomata these plates are 

 present, and in another suborder, the Ctenostomata, 

 they are also present, but generally at the base of the 

 cell. In the living Polyzoa these communication- 

 pores are essential so as to allow the passage from cell 

 to cell of very fine cords, which Joliot provisionally 

 terms endosarc, the special function of which is sup- 

 posed to be of a generative character. Fritz Miiller 

 was the first physiological writer who attempted an 

 "interpretation of this element structure," He 



