STRUCTURE AND LIEE-HISTORY OF A SPONGE. 
23 
granular protoplasm, from which are produced a number of fine 
irregularly branched filaments, which anastomose with similar 
filaments from similar adjacent cells. In general terms, therefore, 
this tissue may be said to consist of a network of protoplasmic 
cells immersed in a quantity of clear indifferent jelly : the jelly, 
in all probability, being derived from the contained cells by 
metamorphosis. Altogether the tissue most closely resembles 
the jelly-like tissue of the disc of the medusa, or jelly fish, and is 
also related to the embryonic connective tissue of the higher 
animals. 
The matrix of the mesoderm serves as a medium for two 
other forms of cells, in addition to the stellate, or branched ones ; 
in some parts of it, particularly in the neighbourhood of the gastral 
ostia, the stellate cells pass into others of a simpler form, by losing 
their branching processes, and becoming fusiform. The fusiform 
cells, so produced, are of considerable length, and lie in parallel 
bundles concentrically round the edges of the gastral ostia ; and 
since they have the property of shortening in the long direction, 
and broadening in the transverse direction, under the influence of 
a stimulus, they serve as muscular sphincters to the ostia, closing 
them when irritated and opening them again on relapsing into 
their normal state. The third kind of cell found in this tissue 
is probably the least differentiated, and is possibly the parent 
form of the others, it is a little mass of protoplasm, containing a 
nucleus and nucleolus, and closely resembles an ordinary amoeba, 
or nucleated blood corpuscle, thrusting out pseudopodia in various 
directions during life, and moving freely by their means through 
the surrounding medium. 
The innermost layer, or endoderm forms a continuous lining 
to the inner wall of the radial tubes, meeting at the edges of the 
pores and gastral ostia, the outer layer of plate-like epithelium or 
ectoderm.’- It consists of more or less flask-shaped cells, each 
^ I have followed Hackel in referring the lining membrane of the stomach 
