xl INTRODUCTION. 



granular mass; and this, by the rearrangement of its 

 material, and further development, is transformed into the 

 elongate and somewhat conical embryo known as the 

 planula. (Woodcut, fig. xvi. 1.) When mature, the em- 

 bryo escapes from the reproductive sac into the water, and 

 for a short period enjoys a free and active existence. The 

 centre of the body is now found to be occupied by an 

 elongate cavity ; the walls which enclose it are composed 

 of two layers, the ectoderm and eudoderm ; and the surface 

 is all but universally clothed with vibratile cilia*. After 

 a while the body enlarges towards one extremity, and a 

 thin chitinous film forms over a portion of its surface 

 (woodcut, fig. xvi. 2); the movements become sluggish ; and 

 at length the cilia disappear altogether, and the embryo 

 fixes itself by the enlarged end, which expands into a flat, 

 circular disk, the remainder of the body standing erect in 

 the centre of it. (Woodcut, fig. xvi. 3.) The disk, by which 

 the embryo is now permanently attached, soon breaks up 

 into a number of lobes, which again divide dichotomously. 

 (Woodcut, fig. xvi. 3 a.) The whole structure is at this 

 stage invested by a chitiuous envelope or polypary. 



As development proceeds the upper extremity is moulded 

 into a polypite within a transparent urn, the lid of which 

 it pushes off when mature. From this primary stem, with 

 its single polypite, by a series of successive buddings the 

 complex plant-like structure is evolved ; while the discoid 

 base gives off the delicate threads that net the surface of 

 weed or stone, and originate and hold together in organic 

 union whole forests of tree-like shoots. 



* The planule of Coppinia arcta, in many respects an anomalous species, 

 is not ciliated. The embryos of Coryne vaginata are unciliated amoeboid 

 bodies, which undergo remarkable changes of form after liberation. 



