PROGRESS OF MICROSCOPICAL SCIENCE. 255 
larva. It lias its greatest development in those sponges whose 
mesodermic products are most abundant (in the spiculated sponges, 
for example) ; it is reduced in the sponges without spicules (for 
instance, Halisarca). The formation of spicules furnishes a new 
example of Heterochrony (heterochronie). They are formed in the 
Halichondrida before the animal has become fixed ; they do not appear 
till after fixation in the Calcispongice ; at least it is so in the normal 
condition. The generality of the appearance of spicules with a ray 
before that of spicules with many rays has a certain importance in 
the history of genera. The cellules of the larva which form the 
exoderm of the sponge are long, transparent, and ciliated ; they form 
in the various groups the anterior part of the embryo; the elements 
which represent the two other leaflets diff'er more generally according 
to the species. Thus in the Calcispongice they are composed of large 
and rounded cells, in the 3Iyxospongi(E the cells are prismatic with 
short flagella ; in the Halichondrida they are united in a continuous 
Plasmodium. In these last sponges these leaflets are produced by a 
delamination of the internal part of the larva ; in the other groups 
they are produced by a direct diff'erentiation of the posterior part of 
the larva. But in both cases the result is the same, thanks to the 
extension outwards towards the posterior part of the larva, of the mass 
of the internal laminas or leaflets (feuillets). The fixation of the larvae 
takes place by their posterior part, i. e. by the laminee which 
normally form this part. At this period the young sponge is, in 
the different groups, a compact mass composed of two superimposed 
laminae, the outer one representing the exoderm, the inner one 
representing the reunion of the internal and middle laminae ; the 
different groups are only distinguished by their sj^icules. The young 
fixed sponge difiers from the larva only by its flattened and irregular 
form. The first phenomenon presented by this young sponge is the 
separation of the inferior lamina into endoderm and mesoderm. This 
phenomenon manifests itself by the appearance of special endodermic 
elements circumscribing a peculiar system of cavities. This is the 
system of endodermic cavities, the most important of these systems in 
the classification point of view. It is represented by the vibratile 
bunches of Leucon and Halichondrida, and by the radiating vibratile 
tubes of Sycon. There are then many other systems of cavities in 
young sponges. One of them, which the author terms a system of 
mesodermic cavities" is a system of canals within canals. It is 
produced by the formation of irregular vacuoles which extend into 
the mesoderm between the vibratile organs. A third system of 
cavities is that in which the sponge takes part in its entirety. Of 
this we see examples in the Sycon, Poterion, Veluspa, and other 
silicious sponges of the cup form. A fourth system of cavities is 
that which is formed by the incomplete fusion of various portions 
of the polypite itself. The importance of characters being regulated 
by the order of their appearance in the embryo, the most important 
character for the natural classification of adult sponges is considered 
by M. Barrois to be the spicules. The disposition of the first system 
of cavities comes next in order. Then follow afterwards the appear- 
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