'Mode of Circulation in the Spongida. 121 



the direction of the fibres and the excretory canals is across or 

 perpendicular to the planes of the wall ; while in Hircinia 

 solida it is the opposite, that is, they are longitudinal or 

 parallel to them. Hence in the former the great excretory 

 canal runs from the pore-surface or subdermal cavities on one 

 side to the vents on the other, much as the " chamber " in 

 Teichoaella labyrinthica ; while in Hircinia solida the great 

 excretory canals run along the whole length of the sides just 

 under the subdermal cavities, more like those in Teichonella 

 prolifera, finally opening at the large vents in the upper end or 

 border ; but in their course sending off in both instances small 

 canals transversely into the sponge-structure (ex. gr. PI. IV. 

 fig. 5, e). Thus, in both cases also, the great excretory canals 

 first receive the water and its nutrient particles from the pores 

 through the subdermal cavities, when the latter, at least, are 

 deflected through the small lateral canals just mentioned, to 

 the spongozoa and ampullaceous sacs. From this point to 

 the ampullaceous sacs they have yet to be traced. 



Although it may be inferred that eacli pore-hole in the 

 ampullaceous sac of Spongelia avara, as represented by 

 Schulze (I. c), is connected with an afferent, tubular canal, 

 this is only represented in that of Halisarca lobularis (l. c. 

 Taf. ii. fig. 13), but its further continuation (that is, its con- 

 nection with the branch coming from the great excretory 

 canal) has yet to be shown ; although the efferent opening in 

 the ampullaceous sac, which is connected with the excretory 

 canal, on the other side, was verified by myself, as above 

 stated, from the commencement (' Annals,' I. c). 



We might now appropriately consider how the rush of water 

 through the great excretory canals is produced, viz. whether 

 it is effected by the cilia of the epithelial cells only, or by 

 some other means, or by both. The other means to which I 

 allude is the presence of circular folds more or less extending 

 round the surface of the canal, which are not induced by the 

 presence of the apertures on the surface, since they often occur 

 without this, as may be seen by the illustrations of Grayella 

 cyathophora and Aacos sjnnipoculum respectively ( c Annals,' 

 1869, vol. iv. pi. vii. fig. 5, g, k, i } and 1879, vol. iii. pi. xxv. 

 fig. 5), while, of course, the presence of the folds alone would 

 be no indication of their having any effect upon the circula- 

 tion, had it not been proved by dissection in the spirit-pre- 

 served specimen of Axos spinijpoculum ) wherein the excretory 

 canal-system is strongly developed, that the wall of the 

 excretory canal is composed of two layers like that of the 

 human intestine, viz. an epithelial layer of cells (/. c. pi. xxv. 



