800 THE ANIMAL KINGDOM. 



Dujardiniy extremely so in Chalinula jer tills. The diameter of the embryo 

 increases during segmentation, and when fully grown it passes into the 

 gastric cavity and escapes by the osculum or by the inhalent canals. The 

 parent sponge may then perish (Chalinula fertilis). 



The free larva has one of three forms, (i) It is an oval blastula with 

 a large blastocoele. The cells are all similar, and the blastocoele is soon 

 filled by the immigration of cells at all points as in Ascetta (Leiicosolenia) 

 primordialis, or at one pole as in A. (L.) blanca and A.(L.) clathrus among 

 Calcarea Homocoela. The cells are collared and flagellate, and one pole 

 is invaginated to form a gastrula in Oscarella lobularis. Or the posterior 

 pole is a patch of rose-coloured non-ciliated cells as in Verongia (Aplysillal) 

 rosea 1 . (2) It is an oval amphiblastula, one half composed of large granular 

 ectoderm cells, the other of columnar ciliated endoderm cells 2 , jointly 

 inclosing a blastocoele. This larva occurs in all Calcarea save the three 

 above-named and in the horny sponge Gumminea (Halisarcat) mimosa. 

 In Sycandra (Sycon) raphanus the granular cells are derived by the growth 

 of a circle of eight cells at one pole of the embryo after it has reached 

 an advanced stage of segmentation (48 blastomeres) : and the columnar 

 cells are invaginated after a period of free larval existence, giving rise 

 to a gastrula 3 . (3) It is solid, either oval or conical, with a posterior 

 flattened or concave pole : its superficial cells are columnar, ciliated, 

 disposed in a single layer, and form the ectoderm of the sponge. They 

 inclose (i) a mass of cells projecting posteriorly as in Chalinula fertilis, 

 Esperia Lorenzii, Clione stationis (?) ; (ii) a mesoglaea with cells, all stellate 

 as in Aplysilla sulfurea, stellate in the centre, hour-glass shaped at the 

 periphery as in Spongelia pallescens ; (iii) cells each contained in a clear 

 capsule, the protoplasm reduced to a stellate figure by an accumulation 

 of a clear liquid as in Euspongia ; (iv) a granular material (coenoblastem 

 of Marshall) with a few nuclei and subsequently many cells as in Plakina 

 monolopha^ or many nuclei as in Reniera filigrana and apparently many 

 other allied sponges. The origin of the contained nuclei or cells is not 

 known except in Euspongia, where cells immigrate inwards during the 



1 Halisarca Dujardini is said by Barrois to have a blastula ; by Metschnikoff to have a solid 

 larva with cells arranged in rosettes, and perhaps derived by immigration. H. pontica has solid 

 larvae, and two or three of them may fuse to form a zygoplanula. 



2 These ciliated cells are said to be collared : O. Schmidt, Z. W. Z. xxv. Suppl. pp. 129-30 ; cf. 

 Haeckel, Kalkschwamme, i. p. 335. 



8 The opposite, i. e. invagination of the granular non-flagellate cells, was said by Keller to 

 happen in Ascandra (Leucosolenid) Lieberkiihnii and Leucandra (Leuconia) aspera, but the latter 

 agrees with Sycandra (Sycon} raphanus, according to Metschnikoff. Barrois concluded that the 

 granular cells gave rise to the endoderm in Ascandra (Leucosolenia) contorta, Sycandra (Grantia\ 

 compressa, S. coronata (Sycon coronatunt] and S. ciliata (Sycon ciliatum), but he observed neither 

 invagination nor the act of fixation. Schulze states that abnormal or transitory invaginations of the 

 granular cells, giving rise to a pseudo-gastrula, may occur in Sycandra (Sycon} raphanus : they 

 are occasionally of great extent but have no significance in development. See Z. W. Z. xxxi. 

 p. 266. 



