MATERIALS FOR A MONOGRAPH OF THE ASCONS. 477 



(b) Anatomy and Histology of the Genus Clath- 

 rina. — The young sponge, as is well known, has the form 

 for which Haeckel proposed the term Olynthus, a form shaped 

 like an open vase or sac, fixed at the blind extremity, and 

 opening at the other by a large aperture termed the osculum. 

 The wall of this organism is pierced by numerous pores, by 

 which water can pass from the exterior into the large central 

 space, or gastral cavity so-called. The inner and outer sur- 

 faces of the body-wall may therefore be conveniently distin- 

 guished as the gastral and dermal surfaces respectively. 



The body-wall is composed of two layers, an outer dermal 

 layer and an inner gastral layer. The former makes up the 

 greater part of the wall, consisting mainly of a structureless 

 jelly; it contains the skeleton, the pores, and the nutritive 

 wandering cells, and is covered at all surfaces where it is 

 exposed by a flat contractile epithelium. The gastral layer is 

 uniform in composition, and consists of the characteristic 

 collar-cells ; it lines the interior of the so-called gastral cavity, 

 but, besides being interrupted at the pores, does not reach the 

 extreme margin of the osculum. There is thus formed a rim 

 or collar surrounding the oscular opening, composed of the 

 dermal layer alone, and covered on both faces by the flat con- 

 tractile epithelium. For this region I propose the distinctive 

 term " oscular rim." 



The olynthus is only a transitory stage in the life-history of 

 a calcareous sponge, and in the Ascons, in which the gastral 

 layer remains continuous and not restricted to '^ ciliated 

 chambers," the complicated form by which the full-grown 

 specimens are characterised is attained by growth at two 

 points. The olynthus increases in length and becomes tu- 

 bular by growth at the oscular rim, and at the same time the 

 surface of the wall is increased by the formation of blind out- 

 growths or diverticula from the sides of the tube. These 

 diverticula continue to grow in length, and repeatedly branch 

 and anastomose until a dense network of tubes, from which 

 new oscula may arise, is formed surrounding the original 

 osculum of the olynthus, which in its turn may have multi- 



