CALCAREA HETEROCOELA 



191 



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

 sponge with a canal system of the type seen in Zeucandra 

 among British forms, but more diagram matically shown in the 

 foreign genus Leucilla (Fig. 

 85). The foregoing remarks 

 do not pretend to give an 

 account of the transition from 

 Sycon to LeTicilla as it occurred 

 in phylogeny. For some in- 

 dication of this we must await 

 embryological research. 



In Lmicandra 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. —Transverse section through the 

 size and rounded form of its body -wall of GranUopsis. rf.o Dermal 



ostium ; fl.cK nagellatea chamber ; i.c, 

 chambers. This change of long mcnrrent canal traversing the thick 



form marks an advance in '^"^'^^ *" Jff' ^^ f^^'^^"'' ^"^^^ ' ^' 



apopyle. (After Dendy.) 



efficiency ; for now the flagella 



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 

 d.o „ J^ 



and proportionately 

 less intense (see p. 

 236). 



Above are de- 

 scribed three main 

 types of canal system 

 — that of Homocoela, 

 of Sycon, and of 

 Leucandra and Leu- 

 cilla. These are con- 

 veniently termed the 



Fig. 85.— Transverse section through the body -will of first, SeCOnd, and 

 Leucilla. (?.o. Dermal ostium ; ea;.c, exhalant canal ; ^j^jp^j tvpes respCC- 

 /Z.c/i., chamber : i.c, inhalant canal. (After Dendy.) J r r 



; tively, and may be 



briefly described as related to one another somewhat in the 

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



