1884.] The Larval Theory of the Origin of Cellular Tissue. 461 
The three layers can be exactly compared with the three layers 
of all the Metazoa, the outer, the ectoderm, is essentially a pro- 
tective and respiratory layer, the inner, the endoderm, is devoted 
in Ascones to digestion, or rather consists wholly of feeding 
zoons, the central, the mesoderm, is a supporting layer secreting 
the primitive spicular skeleton, as in Hydrozoa, Actinozoa and all 
Metazoa. This skeleton, in its spicular character, is similar to 
the primitive condition of the skeleton in most and perhaps 
throughout all the Metazoa, the mode of formation being essen- 
tially at first spiculous and then solidified through the binding to- 
gether of the loose elements remotely comparable with the spic- 
ules of Porifera. It will be noticed also that in sponges the pro- 
ducts of the mesoderm are largely inorganic, as in the skeleton 
in the Metazoa, and the products of the ectoderm are also horny, 
asin the Metazoa. The skeleton of Keratosa, the well known 
webs of elastic threads, which compose the masses known as com- 
mercial sponges, are not products of the mesoderm, but really 
built by the ectoderm. 
The process of digestion in Ascones and in the ampulle is 
purely protozodnal. The cells capture food, living and in an un- 
prepared condition, from the water coming in through the pores. 
They must be admitted to have retained both the structure and 
the habits of the flagellate Protozoa, as has been shown by Clark, 
Carter and Saville Kent. They are really a distinct branch on 
the lower borders of the Metazoa, derived from the Protozoa, and 
equal taxonomically to either Hydrozoa or Actinozoa, forming in 
fact parallels to the first in their development of the ccelomatic 
cavity and gastrovascular system, and even uniting, in this respect, 
Ps two Systems of the Echinodermata, the water and gastric sys- 
=, into one system with two sets of tubes. They have, in fact, 
the incurrent or supply system formed by the ectoderm connect- 
"E with the cloacal outlet system formed by the endoderm in all 
fleshy sponges, Sycones and Leucones. 
— Sponge may, however, be morphologically considered, 
reduced to its simplest elements, as a sac of collared and 
fagellated cells surrounded by the mesoderm and inclosed in the 
oderm. This sac has but one opening, an excurrent and in- 
tive on perture, but this opening is not the blastopore or primi- 
at the a of the gastrula stage. It occurs by a breaking through 
* Opposite pole of the body after the attachment of the 
