PORIFERA. 147 



This feature is also characteristic of the embryos of Spongelia and 

 Euspongia. 



The embryo of the Gummineae (Gummina mimosa) has been in- 

 vestigated by Barrois (No. 122), and has been shewn closely to resemble the 

 typical larvae of calcareous sponges ; one-half being formed of elongated 

 ciliated cells and the other of rounded granular ones. 



Silicispongiae. The development of marine silicious sponges is but 

 very imperfectly understood. The larvae of various forms Reniera (Iso- 

 dyctia), Esperia (Desmacidon), Raspailia, Halichondria, Tethya have been 

 described. Barrois has shewn that the egg segments regularly and that in 

 the earlier stages a segmentation cavity is present. In the later stages the 

 embryo appears to become solid. Externally there is a layer of ciliated cells, 

 and within a mass of granular matter in which the separate cells cannot be 

 made out. The granular matter projects at one pole, and forms a prominence 

 possibly equivalent to the granular cells of Sycandra. In some forms, e.g. 

 Reniera, the edge of the unciliated granular prominence may be surrounded 

 by a row of long cilia. In later stages the granular material may project at 

 both poles or even at other points. One remarkable feature in the 

 development of the Silicispongiae is the appearance of spicula between the 

 ciliated cells and the central mass, while the larva is still free. 



Professor Schulze has informed me that these spicula are developed 

 in mesoblast cells ; while the horny fibres of the sponge are developed 

 as cuticular products of special mesoblast cells (spongioblasts). 



The attachment and accompanying metamorphosis are so diversely 

 described that no satisfactory account can be given of them. The general 

 statements are in favour of the attachment taking place by the posterior 

 extremity where the granular matter projects. 



Carter especially gives a very precise account, with figures, of the 

 attachment of the larva in this way. He also figures the appearance of an 

 osculum at the opposite pole 1 . 



A very elaborate account of the development of Spongilla has been 

 published in Russian by Ganin, of which a German abstract has also 

 appeared (No. 124). 



The ovum undergoes a regular segmentation and becomes a solid ova 

 morula. An epiblast of smaller cells is early differentiated, and in the 

 interior of the inner cells an archenteron becomes subsequently formed. 

 The inner cells next become divided into an hypoblastic layer lining the 



1 Keller (No. 129) has recently given an account of the development of Halichon- 

 dria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by 

 a partial epibolic invagination, the inner mass of cells remaining exposed at one pole 

 and forming there a prominence, equivalent to the granular prominence in the larvas 

 of other Silicispongiae. The free swimming larva resembles the larva of other Sili- 

 cispongiae in the possession of spicula, etc., and after becoming laterally compressed 

 attaches itself by one of the flattened sides. A central cavity is formed in the interior 

 with ciliated chambers opening into it, and is subsequently placed in communication 

 with the exterior by the formation of an aperture which constitutes the osculum. 



IO 2 



