CALCAREA HETEROCOELA 



91 



tycon itself was derived from the Ascon, we shall then have a 

 sponge with a canal system of the type seen in Leucandra 

 among British forms, but more diagram matically shown in the 

 foreign genus Leucilla (Fig. 



m^B^'Mm 



85). The foregoing remarks 

 do not pretend to give an 

 account of the transition from 

 Sycon to Leucilla as it occurred 

 in phylogeny. For some in- 

 dication of this we must await 

 embryological research. 



In Leucandra the funda- 

 mental structure is obscured 

 by the irregularity of its canal 

 system. It shows a further 

 and most important difference 



from Leucilla in the Smaller FlG . 84. Transverse section through the 

 size and rounded form of its body -wall of Qrantiopsi*. d.o, Dermal 



ostium ; Jl.ch, flagellated chamber ; t.c, 

 Chambers. I his Change OI long incurrent canal traversing the thick 



cortex to reach the chamber layer 

 apopyle. (After Dendy.) 



form marks an advance in 



efficiency ; for now the ilagella 



converge to a centre, so that they all act on the same drop 



of water, while in the tubular chamber their action is more 



widely distributed 

 and proportionately 

 less intense (see p. 

 236). 



Above are de- 

 scribed three main 

 types of canal system 

 that of Homocoela, 

 of Syco7i, and of 

 Leucandra and Leu- 

 cilla. These are con- 

 veniently termed the 



Fig. 85. Transverse section through the body -wall of first, SCCOlld, and 

 Leucilla. d.o, Dermal ostium; ex.c, exhalant caual ; j.1- j f-i-npo r p<?npp 

 Jl.ch, chamber; i.c, inhalant canal. (After Dendy.) tIlim L JP bb ieb F^" 



tively, and may be 

 briefly described as related to one another somewhat in the 

 same way as a scape, umbel, and compound umbel among 



