CALCAREA HETEROCOELA 



191 



Sycon itself was derived from the Ascou, we sliall then have a 

 sponge with a canal system of the type seen in Lcucandra 

 among British forms, but more diagrammatically shown in the 

 foreign genus Lmixilla (Fig. 



Transverse section through the 

 body -wall of Gmntiopsis. d.o, Dermal 

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

 long incurrent canal traversing the thick 

 cortex to reacli the chamber layer ; ji, 

 apopyle. (After Dendy.) 



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 



enibryological research. 



In Lcucandra 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 y\g. 84 



size and rounded form of its 



chambers. This change of 



form marks an advance in 



efficiency ; for now the Hagella 



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 

 j||p / fO"" Y^ %\ ' , ^^ types of canal system 



'■"^■*'' fcii^K ; ■•'\ — that of Homocoela, 



of Sycon, and of 

 Lcucandra and Leu- 

 cilla. These are con- 

 veniently termed the 



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

 Leucilla. <;.o, Dermal ostium ; f.c.c, exhalant canal ; ^i ■ i f-i^-^pc rPQnPP 

 A c/i, chamber; if, inhalant canal. (After Dendy.) tllUU L} pes rcbpec- 



tively, and may be 

 Ijriefly described as related to one another somewhat in the 

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



