126 
PHYLUM PORIFERA—SPONGES. 
fact, the sponges are very different Jrom other Metazoa, and 
represent a cul de sac in evolution. 
Budding is very common, 
and in a few cases buds are 
set adrift. Both hermaph- 
rodite and unisexual forms 
occur. The sexually-produced 
embryo ts almost always 
developed within the mes- 
oglea, and leaves the sponge 
as a ciliated larva. With 
the exception of one family, 
Fic. 58.—Sponge spicules. 
1, Monaxon ; 2, triod; 3, triaxon; 4, 
tetraxon 3 5, anchor 2 6, polyaxon ; 
7, a kind of amphidisc, 
all are marine. 
Description of a simple 
sponge. — A 
sponge, such as Ascetfa, is 
very simple 
a hollow vase, moored at 
one end to rock or seaweed, with a large exhalant aperture 
at the opposite pole, and with 
numerous minute inhalant pores 
penetrating the walls. In the 
calcareous sponges, the pores are 
minute perforations in single cells 
(porocytes). 
The walls consist of—(1) a flat 
covering layer; (2) a mesogloea 
containing triradiate calcareous 
spicules, phagocytes, and reproduc- 
tive elements; and (3) a layer lining 
the central cavity, and composed of 
collared flagellate cells, like some 
of the monad Infusorians (cf. Fig. 
55): 
More complicated forms.—But a 
description of a simple sponge like 
Ascetta conveys little idea of the 
structure of a complex form such as 
the bath-sponge (Zuspongia). Let us 
consider the origin of complications. 
Fic. 59.— Section of a 
sponge.—After F. E. 
Schulze. 
Showing inhalant 
canals, 
flagellate chambers, a 
gastrula forming in the 
mesogloea, etc. 
(a) Sponges—long regarded as plants—are plant-like in 
being sedentary and passive. 
They seem also to feed 
