Morphological and systematic studies on the cheilostomatous Bryozoa

to the two last mentioned writers, Calvet and Harmer look upon calcification as a cuticular formation, but while Calvet' thinks that calcification at any rate in the Cheilostomata takes place through the whole thickness of the cuticle, the following observation of Harmer^seems to suggest that lie is inclined to share Nitsche's view of tlie calcification as proceeding in the central part of the cuticle: »hi incinerated specimens the lateral walls of neighbouring zocecia may appear separated from one another by a narrow slit in place of the '>raised line«.This is in fact the edge of a chitinous layer separating contiguous zocecia, and prolonged into the membranous epitheca.This agrees with the account given by Nitsche of the calcification of the zocecia of Membranipora membranacea, in which calcareous matter is said to be formed in the middle of the chitinous ectocyst, part of which is left on each side of it«.We shall return later to this statement.As I have not been able to examine living material I do not consider myself qualified definitely to decide which of the views is the right one, still it seems to me that the »ceU-theory« is the one which explains the different phe- nomenona, which the calcification presents, in the easiest and most natural way, and it seems to me especially difficult to explain the presence of such solid spinous processes on the outer surface in a number of species (e. g. in Holoporella columnaris) as well as on the inner (e. g. in Menipea roborata Hincks and M. ligulata Mac Gill.) by the aid of the cuticular theory.We may now consider a number of differences which the calcification pre- sents, and to begin with we may distinguish between more or less compact or firm calcifications.The veiy different resistance whicli the calcified skeleton is able to offer against breaking and grinding shows sufficiently that the compactness and firmness can be different, and the firmest skeleton is undoubtedly found in the families of Sclerodomidae and Reteporidae just as we find tlie weakest in the families Bicellariidae, Flustridae, Onchoporidae and in certain species of Membranipora.If we regard parts of the skeleton of certain, very slightly calcified species (e. g. of Membranipora membranacea, Electra pilosa, Flastra carbasea, Dendrobeania murray- ana and Onchopora Sinclairi) under a rather high magnification, it shows a grained or dotted appearance, but under a very high power (immersion) it dissolves into a dendritic network, the meshes of which enclose numbers of small uncalcified spots, which give a reddish light.Sometimes, however, the same wall may show more or less calcified parts.We thus find in Dendrobeania murrayana that the part of the basal wall, wiiich touches the distal wall, is much more calcified than the other part, and in the middle of the basal wall in Escharoides Jacksoni ' 9, pp. 29& 165; ^17, p. 227.1* Waters we find a large oval white spot which is less calcified than the rest of the hasal wall, being formed by a net-work of meshes, and the white colour is due to the fact that the light is reflected from the numerous small surfaces of which this net-work is composed.In Membraniporina arctica, Smittina trispinosa, var.lamellosa and Sm.propinqua the basal wall is covered by small snow-white, round spots of a similar structure.As to the manner in which the calcification takes place, we can distinguish between compound and simple walls, as a wall in some cases calcifies as a whole, while in other cases it calcifies in more or less separated pieces, which at any rate up to a certain period are separated from one another by sutures, and these sutures are in some cases very distinct for a long time, while in other cases they disappear verj^quickly.This concerns very often the frontal wall, and is due to the circumstance that this is very often provided with covering layers or sculpture of various kinds.As examples of species with simple walls we may mention Membranipora membranacea, Electra pilosa, species of the genera Onychocella, Hippothoa, Thalamoporella, and Steganoporella and also -it seemsall members of the families Bicellariidae and Scrupocellariidae.The parts into which a compound wall can be divided vary very greatly in size, and the smallest of them give the impression, not only from their exceedingly small size, but also from their shape, that they are calci- fied cells, as they have the same crenulated contour which as a rule seems to distinguish the ectoderm cells in the Bryozoa.This form of calcification, which we might call >cell-mosaic« is for instance found on the basal wall in Membrani- pora Savarti, Flustra denticalata (PI.XIX, fig. 10 c), Porella concinna, Hippopodina feegeensis Busk, Smittina Lansborovi, as well as on the lateral walls of Flustra sermlata.Gradually several of these very small cellular areas fuse together to larger ones, and in older zooecia the mark of division may quite disappear.In contrast to the very fine mosaic we find in the jtist-mentioned species, other species pre- sent a mosaic consisting of much larger but still comparatively small areas, which cannot very well be regarded as cells.This form of calcification, which we might call »plate-mosaics we find very finely developed on the basal wall of Flustra securifrons (PL XIX, fig.8 a) and we may here give a detailed description of its appearance in this species, which like most Flustra species has a perfectly uncalcified frontal wall.The composition of the different walls can best be seen, as everywhere in the Bryozoa, after they have been boiled for some time in alkali, or have been treated with eau de Javelle, which even in a cold condition has a far better effect than boiling alkali.Having dissolved all organic parts with such treatment, we find as a rule a row of square or hexagonal plates along the middle of every basal wall, while it seems as if a similar longitudinal ' 54, p. 246; 55, p. 3. the surface, all the others end either in a new pore or in another sutural line.In Crisia eburnea the calcification takes place in narrow longitudinal belts, and a similar mode of calcification is seen in the short and wide hollow protuberances, which are situated on each side of the aperture in several Thalamoporella (PI.VI a, figs.4 a,^5 a).Besides the form of striping, which is due to lines of growth, and which for instance is often very distinct in the Hippothoa species, the basal wall especially of the zocecia often presents a distinct, longitudinal or fan-shaped striping which is most probably due to the arrangement of the separate lime particles.This form of striping is widely distributed in the species BicellariidcE and Scrupocellariida;.Before leaving this subject I must shortly mention an apparent observation made by Nitsche', according to which the calcified frame of every zooecium of Membr.membranacea after boiling in alkali is divided into three pieces, namely, in two double-folded end pieces each consisting of a terminal partition wall and a piece of the adjoining side-wall, as also of two lateral pieces.This view is nevertheless not correct.After boiling like this more or fewer zocecia in a col- ony may indeed show cracks or bendings, but these are quite accidental, and not an expression for the mode of calcification of the zocecia. Cryptocyst and Gymnocyst.Under the generic name Onychocella JuUien^has described several recent species of a type which had a great extension in the seas of the chalk period but which only has a small quantitj' of now living representatives.As is the case in a Flustra species, the whole of the frontal wall is covered with a membrane in which can be seen an opercular valve, but when we remove this membranous cover we find underneath it and separated from it by a distinct space a more or less concave calcareous layer, which distally has a semicircular aper- ture (opesia) through which the polypide .can make its way out.This aperture which was formerly regarded as the orifice of the zooecium in the fossil species is consequently separated by a space from the real orifice, which is situated in the covering membrane.In contrast to the membranous ectocyst Jullien designates this deeper-lying calcareous ectocyst as a Cryptocyst, and proposes on the basis of this observation to divide the cheilostomatous Bryozoa into two divisions, according to the presence of a single or double ectocyst.To the first division: Monodennata, he refers such forms as Eschara foliacea, Lepralia hyalina, L. coccinea, Cellepora pumicosa and Flustra foliacea, and to the other division: Diplodermata, besides ' 80, p. 42; " 42. the species of the genus Onychocella, Biftustra delicatnla, Vinciilaria abyssicola, Steganoporella Smitti, etc.In a later paper' besides a number of new genera, species of the genera Aetea, Microporella, Scriipocellaria, Bicellaria, Cibrilina, Lageni- pora, Schizoporella, Smittia, Mucronella and Retepora are referred by him to the Monodermata, while he classes species of the genera Caberea, Membranipora and Setosella to the Diplodermata.In a third paper however he has altered^his view of the extent of the Diplodermata, in that he now classes some of the forms which in the earlier papers he placed under the Monodermata to the Diplodermata, namely, all the families Eiicrateidae, Cellulariidae (= ScrupocellariidaeJ, Bi- cellariidae, Notamiidae, Flustridae, Membraniporidae, Gemellariidae and Farciminariidae.While a systematic classification on the basis of the structure of the frontal wall is still found in a work of Canu^from the year 1900, on the Bryozoa of the Cretaceous period, this classification seems quite given up in a later coopera- tive work by Jullien and Calvet*, which after the death of the first men- tioned writer has been carried on and published by the latter.Calvet-' gives very important information about the structure of the frontal wall in a large work dealing with the structure and development of the ectoproct Bryozoa.Be- sides in Eucratea Lafonti, Membranipora Rosseli and the species of the genus, Cellaria, he has found a double ectocyst in the species examined by himself of the genera Tubiicellaria, Microporella, Chorizopora, Schizoporella, Lepralia, Umbonula, Retepora and Cellepora, consequently in forms which accoi'ding to Julliens ' 30, p. 250.' 8, p. 59; '' 109, p. 280.area, in the same manner as in Escharina, disappears, and as this was the most pregnant character of the section Escharina, here, also, we perceive the close proximity of that group, although yet the plain front side and the raised pri- mary margins of the zocecia remind us of the Flustrine nature*.The answer to the question: whence the Ascophora have originated, is not quite so evident, as we have here a compensation sac, of which organ no trace ' 72, p.