34 STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
spicules to support it, and with three layers of tissue composing 
the wall, the ectodermal covering of plate-like cells, the jelly-like 
mesoderm in the middle and the flagellated endodermal cells 
within. But, as yet, there is no trace of the radial tubes. Some 
sponges ( Olynthus) which have the same developmental history as 
Sycandra, up to this point, remain persistently in the stage now 
reached by it; in the young Sycandra, however, budding now 
begins to take place from the stomach wall, little hollow processes 
jut out from it, as if pushed out by a finger, these grow outwards, 
till they acquire exactly the same characters as the sac from which 
they proceed ; the open gastral ends correspond to the mouth of 
the stomach, and the outer ends of the tubes to the base of the 
stomach. These are the radial tubes, and at first they are 
separate from each other, not united ; this stage in the history 
of the sponge remains permanently throughout life in a related 
species (S. coronata) ; in our sponge, however, they soon become 
united by transverse bars of tissue, which cross from one tube 
to another. The ends of the tubes, however, always remain free 
as little conical protuberances, but in another sponge (S. capillosa) 
development proceeds the one step further, and the tubes become 
joined right up to their extremities. The fact here illustrated, 
that a stage which is transitory in the history of one animal 
is persistent in another, is one of the strongest arguments ^for 
Darwin,’ 
After so much pure description one may fairly be allowed to 
indulge in a little speculation ; at one time people who thought 
at all about the matter were accustomed to believe that the young 
animal was produced from the adult all at once, at a single 
stroke ; it commenced as a minute germ, a more or less exact 
likeness of the parent in miniature, which had nothing to do 
except to grow big ; such, however, is as we can see, the very 
opposite of being the case, a vast number of phases of develop- 
ment intervene between the fertilized ovum and the young 
