PORIFERA. 



145 



mode of development with Sycandra, nevertheless the concordant researches 

 of O. Schmidt (No. 138) and Metschnikoff (No. 134) have shewn that this 

 is not true for the genus Ascetta (As. primordialis, dathrus and bianco). 



The larvae of these forms are very differently constituted to those 

 of Sycandra. 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 large granular cells assume a radiate arrangement round a central 

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

 cells become placed between the epiblast and hypoblast and constitute the 

 mesoblast. 



Myxospongiae. In this group Halisarca has been investi- 

 gated by Carter (No. 123), Barrois (No. 122), Schulze (No. 141) 

 and Metschnikoff (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 seg- 

 mentation cavity is early formed, which is never, as in Calci- 

 spongise, 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 

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

 B. II. 10 



