PORT F ERA. 119 



The larvae of these forms are very differently constituted to those 

 of Sycaudra. They have an oval form and are composed of a single 

 row of ciliated columnar cells : their two extremities only differ in the cells 

 at one extremity being longer than those at the other. Especially at the 

 pole where the shorter cells are situated (Schmidt) a metamorphosis of the 

 cells takes place. One after the other they lose their cilia, become granular, 

 and pass into the interior of the vesicle. Here they become differentiated 

 into two classes (Metschnikoff) ; one of larger and more granular cells, 

 and the other of smaller cells with clearer protoplasm. Cells of the former 

 class are mainly found at one of the poles. When the larva becomes 

 free the cells in the interior of the vesicle increase in number and nearly 

 fill up its central cavity. After a short free existence the larva becomes 

 fixed, and the epiblast cells lose their cilia and become flattened. At a later 

 period the lai-ge granular cells assume a radiate arrangement round a central 

 cavity and become clearly marked out as the hypoblust cells. The smaller 

 cells become placed between the epiblast and hypoblast and constitute the 

 mesoblast. 



Myxospongiae. In this group Halisarca has been investigated by 

 Carter (No. 123), Barrois (No. 122), Schulze (No. 141) and Metschni- 

 koff (No. 134). The ova develop in the mesoblast, and when ripe 

 occupy special chambers lined by a layer of epithelial cells. Schulze 

 has found the spermatozoa of this genus of sponge and has been able 

 to shew that the sexes may be distinct, though many species of 

 Halisarca are hermaphrodite. 



The segmentation is, roughly speaking, regular, and a segmentation 

 cavity is early formed, which is never, as in Calcispongiae, open at 

 the poles. When the larva leaves the parent it is an oval vesicle 

 formed of a single layer of columnar ciliated cells. Slight differences 

 may be observed between the two extremities of the larvae of most 

 species. One of these the hinder extremity is directed backwards 

 in swimming. 



The further history of the larva has been investigated by 

 Metschnikoff. He has found that the interior of the vesicle becomes 

 gradually filled with mesoblast cells of a peculiar type, called by him 

 rosette-cells, which are probably derived from the walls of the vesicle. 



When the metamorphosis commences, the larva assumes a flat- 

 tened form, and cells of a new type, viz. normal amoeboid cells, 

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

 derived from the epiblast. The larvee 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 esta- 

 blished independently. 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 com- 

 munication with the exterior. 



