SPONGES 



75 



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

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

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

 follows : 



Posterior Granular Cells. 



1. Present 



2. Absent 



3. Absent 



Immigration. 



Multi polar (Ex. Cl. blcmca}. 

 Multipolar (Ex. Cl. cerebrum] 

 Unipolar (Ex. Cl. ref.iculum). 



Fiu. 59. 



Types of sponge larva*', diagrammatic ; the ciliated cells are left clear, the dermal cells 

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

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

 newly-hatched larva of fsucosolenifi (or pseudo-gastrula stage of Sycon); 3, late larva of I^euco- 

 snlenia (or newly-hateliel larva of Sycon) ; 4, larva of Oscarclla (ai'ter Maas) ; archaeocytes 

 conjectural ; 5, larva of Mt/.rllln (after Maas) ; 6, completely ciliated larva of a horny sponge ; 

 SfMgtUa is similar, but contains a eavity near to the anterior pole. 



The type of parenchymula larva exemplified by Clathrina 

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

 amphiblastula larva found in Leucosoleniidae, and in the great 

 majority of Heterocoela. To understand the evolution of this type 

 it is necessary to suppose that in a normal parenchymula larva 

 with archaeocytes placed internally, and with immigration at the 

 posterior pole, the segmentation cavitj" 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 



