478 E. A, MINOHIN. 



plied by constriction and subsequent fission of the tube in a 

 longitudinal direction. Thus arises the sponge form typical 

 of the Ascons, consisting of a dense network of hollow tubes 

 converging towards, and opening by, one or more oscula ; and 

 we obtain either the large erect oscular tubes with com- 

 paratively small basal network characteristic of the genus 

 Leucosolenia, or the short and often insignificant oscula, 

 acting as vents for a greatly developed network of tubes, cha- 

 racteristic of the genus Clathrina, according as growth pre- 

 ponderates at the oscular rim, or at the ends of the diverticula. 



Such being the general structure of the sponge, we may now 

 consider in more detail the composition of the body-wall, and 

 more especially of the dermal layer. The gastral layer is 

 uniform throughout, and consists only of collar-cells. For the 

 purposes of the present investigation it is not necessary to 

 discuss the details of the structure of the collar-cells, since 

 they take no share whatever in the formation of the spicules. 

 Onlv one point may be mentioned, sufficiently obvious when 

 attention is called to it, but which, if passed over in silence, 

 might lead to the impression that my figures or preparations 

 were at fault. The wall of the sponge is the wall of a hollow 

 cylinder, and is possessed of a certain thickness ; hence the 

 gastral layer, which occupies a more internal position, has a 

 less extended surface than the more externally placed dermal 

 layer. As a consequence when the wall is laid out flat the 

 gastral surface is stretched, and the collar-cells tend to separate 

 slightly from one another here and there, producing cracks 

 and spaces, as can be seen in PI. 41, fig. 39, and PI. 42, fig. 50. 

 These spaces are not natural, but are the inevitable result of 

 flattening out a curved surface. In their natural position the 

 collar-cells are in close contact, their cell limits forming in 

 surface view a network of polygonal areas. 



The dermal layer in the genus Clathrina is sharply 

 differentiated into an external contractile or neuro-muscular 

 layer, and a more internal connective-tissue layer, scattered 

 about in which are the pore-cells and the wandering cells. 

 The connective-tissue layer consists of spicules and their 



