SPONGES 



129 



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

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

 we have seen, there is no ectosome (Fig. 6 1,/). In Plakina dilopha, 

 however, the 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 cortex, 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 trilopha 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 with the 

 subdermal cavities which, as we 

 have 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, 7J). 

 In such forms the incurrent canal 

 system may commence with an 

 arrangement known as a clume (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 III the COrteX, 

 ' ' 



FIG. 87 



Section through ?he eortex of c ydonium 



eosaster, Soil., .showing .the pore sieve over- 

 lying the chone, which communicates 

 and therefore Comparable tO a SUb- through a sphinctrate aperture with the 



dermal cavit. The ectochone subcor1 



cavity. 



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 described, 



with its flagellated "chambers. The dotted 

 circles in the cortex are sterrasters con- 

 nected by fibrous strands. (After Sollas, 

 "Challenger" Report, xTb.) 



From the latter come off 



the subcortical crypt 



