SPONGES 



75 



tinually modified and pass iuto the interior, their place being filled by 

 the closing in of the ciliated layer. Thus three types of iiarenchynmlae 

 can be distinguished in the Clathrinidae, which may be tabulated as 

 follows : — 



Posterior Granular Cells. 



1. Present 



2. Absent 

 :i. Absent 



Immigration. 

 Multipolar (Ex. CL blanca). 

 -Multijiolar (Ex. CI. cerebrum] 

 Unijiolar ^Ex. CI. reticulum). 



Types of sponi,'f larvae, diagrammatic ; tlie ciliated cells are left clear, the deniial cells 

 (inner mass) are shaded, the archaeocytes are granulated. Transformation of ciliated (gastral) 

 into dermal cells is represented by graduated shading. 1, larva of Clathrina reticulum ; 2, 

 newly-hatched larva of I.eucosoknia (or pseudo-gastrula stage of .Si/coii) ; 3, late larva of I.euco- 

 solenUi (or newly-hatched larva of Sycon) ; 4, lar\'a of Oi^nmlla (^'ter Maas) ; archaeocytes 

 coiyectiiral ; 5, larva of Mi/xilla (after Maas) ; 6, completely ciliated larva of a horny sponge ; 

 SjKngilla is similar, but contains a cavity near to the anterior pole. 



The type of parenchymula larva exemplified by Clatlirina 

 reticulum (Fig. 59, 1) affords an easy transition to the so-called 

 amphiblastula larva found in Leucosoleniidae, and in the great 

 majority of Heferocoela. To understand the evolution of this type 

 it is necessary to sup])ose that in a normal parench3mula larva 

 ^vith archaeocytes placed internally, and with immigration at the 

 posterior pole, the segmentation cavity has become greatly reduced, 

 and is practically filled up by the archaeocytes. The consequence 

 of this will be that the ciliated cells which become modified into 



8 



