OALCAEEA. 45 



in the middle of a solid jelly. There is no epithelial sack, but the epithelium 

 appears as the lining of the incurrent canals, which are more or less circular pipes 

 through the jelly. The pore cells have not been made out with certainty in this 

 species, but the cells lettered p.c. in Figs. 51 and 53 are probably the pore cells, 

 which of course are closed. 



ApopyleS. — The apopyle usually has the appearance of an oval opening in a 

 thin elastic membrane stretching across the end of the flagellated chamber. It is 

 probable that the apopyles can be closed in all species. 



Excurrent Canals. — In Leucandra gelatinosa the excurrent canals close by the 

 contraction of the epithelial lining. When the canal contracts, the spicules are left 

 in their normal position, and the lining cells stretch the gelatinous mesoderm behind 

 them, so that it is drawn into the canal. The apical rays do not appear to interfere 

 with this action. In cross-section the contracted canal, therefore, appears as an 

 irregular ring of spicules filled up with jelly, in the centre of which lies the contracted 

 group of lining cells (see Figs. 45 and 46). It is probable that similar contractility 

 may be possessed by other species which have a strongly developed mesoderm ; 

 as a rule, however, the excurrent canals do not close. 



Gastral Cavity. — In some Homocoela the gastral cavity can be closed solid.* 

 The author does not know of any instance in which this occurs among the Heterocoela. ■ 



Osculum. — The osculum may be closed in a variety of ways : — 



(1) By simple contraction of the surrounding mass {e.g., Leucandra p^Hmigenia). 



(2) By the folding inwards of the oscular collar {e.g., Leucandra gelatinosa), etc. 



(3) By means of a diaphragm across the mouth {e.g., Megapogon raripilus). 



Diaphragms have been found in several of the new species, but not in good 

 enough preservation to enable their structure or method of closing to be investigated. 



Spicules. 



The dimensions given for the spicules in the detailed descriptions of the several 

 species in Part II. have generally been taken from the drawings, which were traced by 

 camera lucida accurately to scale, and show as far as possible a representative selection ; 

 they must not be understood to be either limiting (maximum and minimum) dimen- 

 sions or accurate average dimensions. To ascertain the true limiting and average 

 dimensions would require a detailed examination of a large number of specimens and 

 great care in the preparation of the spicule slides, in order to insure that all the 

 spicules from each specimen were mounted. There appeared to be no advantage to 

 be gained in the present instance by attempting such accuracy, which, indeed, would 

 only have been possible in the few cases where a sufficient number of specimens 



existed. 



When the facial rays of a triradiate or quadriradiate ^spicule are " folded," i.e., do 



* Vide Minchin (1). 



