PORIFERA. 121 



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 

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

 appeared (No 124). 



The ovum undergoes a regular segmentation and becomes a solid oval 

 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 

 archenteron, and a mesoblastic layer between this aud the now ciliated 

 epiblast. At the narrow hinder end of the embryo the mesoblast becomes 

 thickened, and largely obliterates the archenteron. In this part of the 

 mesoblast silicious spicula are formed. The larva becomes attached by 

 its hinder extremity, and in the course of this process flattens itself out 

 to a disc-like form. From the nearly obliterated archenteric cavity out- 

 growths take place which give rise to the ciliated chambers. These 

 are not placed directly in communication with the exterior, but open, if I 

 understand Ganin rightly, into a space in the mesoblast, which subsequently 

 acquires an exterior communication the primitive osculum. The subse- 

 quent pores and oscula are also formed as openings leading into the meso- 

 blastic cavity, which communicates in its turn with the ciliated chambers. 



It appears that in the present unsatisfactory state of our know- 

 ledge the larvse of the Porifera may be divided into two groups : viz. 

 (1) those which have the form of a blastosphere or else of a solid 

 morula ; (2) those which have the amphiblastula form. 



In the former type the mesoblast and hypoblast are formed either 

 from cells budded off from the outer cells of the blastosphere or 

 from the solid inner mass of cells; while the outer ciliated cells 

 become the epiblast. This type of larva, which is found in the 

 majority of sponges, is very similar in its general characters and 

 development to many Coelenterate planulae. 



The second type of larva is very peculiar, and though in its fully 

 developed form it is confined to the Calcispongia?, where it is the usual 

 form, a larval type with the same characters is perhaps to be found 

 in other sponges, e.g. amongst the Gummineae, and amongst the 

 Silicispongia3 where one half of the embryo is without cilia, though 



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

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

 a partial epibolic imagination, 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 Silicispongise. The free swimming larva resembles the larva of other Silicispongise 

 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. 



