SPONGES 



129 



The species of Phikina, on the other hand, furnish an interesting 

 series of modifications of another type. In riokina monolopha, as 

 we have seen, there is no ectosome (Fig. Gl,/). In riakina diloplut, 

 however, tlie distal extremities of the lobes of the choanosome are 

 greatly thickened over their whole outer surface, and coalesce 

 with one another to form a thick cwtex, traversed by the much 

 narrowed incurrent canals. There are in this case neither 

 dermal membrane nor subdermal cavities, and the ectosomal 

 portions of the incurrent system are no wider, and may even be 

 narrower, than the choanosomal portions. Plakina trilojjha carries 

 this state of things even further, the cortical layer being of greater 

 thickness, and the incurrent canals further complicated by secondary 

 folding of the choanosome. The incurrent canals may widen consider- 

 ably after traversing the ectosome, to form wide subcortical crypts, 

 lying in the choanosome, and therefore not homologous Avitli the 

 subdermal cavities which, as we 

 liave seen, belong to the ectosome. 



The growth of a cortex, so well 

 seen in a simple condition in 

 Plakina, is carried to a high pitch 

 of development in many other 

 sponges, especially in the Tetracti- 

 nellids and their allies. In a 

 typical corticate sponge the body is 

 enclosed in a tough fibrous rind, 

 often fortified by special differentia- 

 tions of the skeleton (Fig. 30, B). 

 In such forms the incurrent canal 

 system may commence with an 

 arrangement known as a clione (Fig. 

 87), which may be taken as typifying 

 the extreme of differentiation under- 

 gone by the incurrent system. The 

 dermal pores (ostia) are grouped 

 to form pore sieves, and perforate 

 a thin membrane which roofs over 

 a funnel-shaped cavity, termed the 

 ectochone, situated in the cortex, 

 and tiierefore comparable to a sub- 

 dermal cavity. Tiie ectochone 

 leads through a narrow aperture, 

 surrounded by a contractile 

 sphincter, into a spacious sub- 

 cortical crypt, termed the endochone 

 the incurrent canals (sensu strictiori). 



Although, in the instances descriljed, the subcortical crypt 





Fig. 87. 



Section throuKli the cortex of Cylnniuvi 

 eogaster, Soil., showiii<; the pore sieve over- 

 lying the chone, which coinnmnicates 

 through a siiliiiictrate aperture wiUi the 

 .subcortical cryj)! , lying in the choanosome 

 with its flageliatfd Vhanihers. The dotted 

 circles in the cortex are stcrrasters con- 

 nected by fibrous strands. (Alter SoUas, 

 " ChuUenijcT " Tieport, x 73.) 



From the latter come off 



