EVOLUTION. ANATOMY, ETC. 
Spong. 3 
dermal cells are transformed into a thin layer of small ciligerous 
cells, forming in the young fixed sponge the superficial or ectodermal 
layer, while the large basal colls form the inferior layer, in which the 
canals and vibratile baskets are developed. The oscula are only formed 
at a late period. In the siliceous sponges examined {Isodictya, Desma- 
cidon)^ 2 i. complete segmentation of the egg takes place; in the larva 
the larger portion of the body is formed of a layer of small ciligerous 
ectodermal cells ; the posterior and the whole interior consists of a con- 
fluent protoplasmic mass ; the limit between this protruding plasmodium 
and the ciliated portion is marked out by a ring of large flagella. The 
larva does not always fix itself by its posterior extremity. The young 
sponge is composed of the same two layers as in other groups : in the 
inferior (mesodermal), the spicules (the development of which begins in 
the larva), the “ vibratile baskets ” and canals are formed ; in the 
superior, the pores and oscula, which are homotypical with the pores, 
but not with the mouth of the Ccelenterata. A close parallelism with the 
Bpongozoa therefore appears inadmissible. 
Keller (6), on the other hand, emphatically denies, in the calcigerous 
and siliceous Sponges, the existence of the layer of plate-like epithelial 
(ectodermal) cells observed by Schulze and Metschnikoef ; the skele- 
togenous syncytium really corresponds with the ectoderm of the larva. His 
embryological observations were made on Calcispongia, especially Sycan- 
dra raphanus. The fecundation of the egg by spermatozoa was observed 
in a single instance. The amoeboid ambulatory cells of the syncytium are 
developed into eggs, but probably originate in the syncytium, and are not 
emigrants from the endoderm. In opposition to the observations of Barrois, 
Keller maintains that a true invagination and formation of a “gastrula ” 
takes place after the liberation of the larva from the ectodermal (or 
mesodermal) tissue and its migration into the tubes or gastral cavity of the 
parent sponge, the dark moiety being invaginated in the ciligerous por- 
tion, constituted by the yellow prismatic cells ; in this manner a stomach 
is formed, and a mouth, which is afterwards obliterated ; the syncytium 
(in which the first spiculae afterwards appear) is formed by fusion (pro- 
gressing from inwards) of the ciligerous ectodermal cells, while the dark 
invaginated cells build up the endoderm. The larva fixes itself by the 
ab-oral pole (this is contrary to the majority of observations) ; before 
the fixation, the cilia become feeble and finally disappear. These results 
are again made doubtful by F. E. Schulze (10), who, after repeated 
investigations, made independently of those of Barrois, has arrived 
precisely at the same results on the main points, the true epoch of the 
invagination and its transitory character, in opposition to his own earlier 
observations and views [Zool. Rec. xii. p. 664]. On the other hand, 
Keller allows that the invagination may (exceptionally) take place 
before the birth of the larva, but the fact that the “ gastrulae ” are found 
swimming freely in the tubes and “stomachs” of the parent sponges, 
proves, in his opinion, that the gastrula-state is later than the oviform 
larva. He never saw the formation of spiculse in the non-ciligerous 
portion of the larva, as asserted by Metschnikoef and Gotte (0. 
Schmidt, 9). 
