E. MARSHALL!. — GEN. AEEAXGEMEXT OF SOFT PARTS. 119 



met with in that region of tlie body, especially in very young 

 specimen?, in which the wall is still thin and backward in the 

 development of its parts. 



As is easily conceivable, the increase of the wall in thick- 

 ness and of the chamber-layer in the extent and complexity of 

 its evagi nations, puts into requisition a freer passage than before 

 for the accelerated in- flow and out-flow of water, and thus arise 

 the incurrent and the excurrcnt canals. Both of these systems 

 of canals are simply relatively larger intertrabecular S2:)aces which, 

 in the form of elongated passages, penetrate more or less deeply 

 into the choanosome. The canals are therefore, at the commence- 

 ment of their formation, indistinguishable from ordinary inter- 

 trabecular lacunas. However, after attaining a certain length 

 and caliber, they deserve their name all the more since the 

 lining trabeculre and certain spicules give to them a more or less 

 definite, though of course much interrupted, septum-like wall. 



TJie excurrent canals (PI. IV, fig. 28, ex. c.) develop each 

 as a direct continuation of the gastral cavity in the axis of the 

 efferent hollows inclosed in the evagiuations, before mentioned, 

 of the chamber-layer. The}" therefore not only correspond in 

 their ^losition with, but also repeat to a great extent the branched 

 configuration, of the latter. The result is that the canals directly 

 communicate with the gastral cavity by widely open orifices, 

 which, unlike those in many species belonging to other families, 

 are not covered over by a continuous endosomal layer supported 

 by a lattice-work of gastralia. The internal trabecular system, 

 forming a thin layer, is directly continued from the gastral 

 surface into the evaginatious of the chamber-layer, along the 

 inner surface of these and around the lumen of the excurrent 

 canals. Toward the ultimate branches of the evaginatious and 



