146 CERATOSPONGI^:. 



cells, grow in amongst the rosette cells. The new cells are also 

 derived from the epiblast. The larvae appear to fix themselves 

 by the hinder extremity. The cilia gradually disappear, and 

 the epiblast cells flatten out and form a kind of cuticle. For 

 some time the larva remains in the two-layered condition, but 

 gradually canals (? ciliated chambers) lined by hypoblast cells 

 become formed. They appear as closed spaces with walls 

 of ciliated cells derived from the amoeboid cells, and the 

 different parts of the system of chambers are established inde- 

 pendently. In H. pontica the ciliated chambers are formed 

 before the attachment of the larva. The development was not 

 followed up to the formation of the pores placing the canal 

 system in communication with the exterior. 



The young sponges at a somewhat later stage have been 

 studied by Schulze and Barrois. They are formed of an 

 external layer of flattened cells, not clearly ciliated as in the 

 adult, within which are a normal mesoblastic tissue, and several 

 spherical chambers lined by ciliated cells exactly like the ciliated 

 chambers of the full-grown sponge. Irregular invaginations of 

 the epiblast give to the young sponge a honeycombed structure. 

 The ciliated chambers in the youngest condition of the sponge 

 are closed ; but in slightly older examples they come into com- 

 munication with the passages lined by hypoblast, and so indirectly 

 with the external medium. 



Ceratospongiae Amongst the true Ceratospongiae the embryos of 

 two of the Aplysinidse, and of Spongelia and Euspongia have been to some 

 extent worked out by Barrois and Schulze. The form worked out by Barrois 

 is called by him Verongia rosca. The segmentation is nearly regular, but 

 from the first the segments may be divided according to their constitution 

 into two categories. At the close of segmentation the embryo is oval and 

 covered by a single layer of columnar ciliated cells ; these cells may however 

 be divided into two categories, corresponding with those observable during 

 the segmentation. A certain number are coloured red and form a definite 

 circular mass at one pole, while the remainder, which constitute the major 

 part of the embryo, have a pale yellowish colour. Those at the red pole 

 lose their cilia in the free larva, but around the area formed by them is a 

 special ring of long cilia. The chief peculiarity of the embryo (made known 

 by Schulze) consists in the fact that the layer of cells which covers the 

 embryo does not, as in other sponge embryos, simply enclose a space, 

 but the interior of the embryo is formed of a mass of stellate cells like the 

 normal mesoblast of full-grown sponges. 



