C(ELENTERATA. 15-3 



in radial symmetiy round the moath. The mouth 

 opens into the oesophagus or gullet, a short tube lined 



spores which bear the former kind are those called microspores, 

 those which bear the latter kind maorospores. This is a step 

 towards the state of things that exists in the so-called flower- 

 ing plants, more correctly spoken of as Phanerogamia, or 

 showy-flowering plants. In these the prothallium is much 

 reduced : the male prothallium is represented by the root- 

 like pollen-tube, which germinates from a pollen grain which 

 has fallen on the stigma of a flower, but no antherozoids are 

 ever produced by it, and the pollen-tube itself passes directly 

 into the seed to fertilize it. The female prothallium is so 

 reduced that it never passes outside the spore-coat, which is 

 represented by a structure called the embryo sac, lying inside 

 the ovule (as the unripe seed is called), and usually gives 

 rise only to a single oosphere. It now exists as the en- 

 dosperm of seeds, or albumen, as it is called, from its usual 

 chemical composition. One of the links by which its real 

 nature is traced is fonnd in the Goniferce, or Pine-tree family, 

 where there are seen in an early stage of the ovule several 

 oospheres in the endosperm of one seed, each surrounded by 

 a group of cells which represent an archegoniura. The endo- 

 sperm persists as the outer substance surrounding the embryo 

 in the kernel of the pine-cone. The substance of this seed, 

 therefore, is the homologue, or representative, of the little 

 green leaf-like prothallium of the fern, which, from being an 

 independent plant, has become reduced to a mere organ of 

 the asexual plant, or sporophore. The endosperm is utilized 

 as a source of nutriment for the embryo in the ripened seed . 

 but in some seeds it becomes obliterated, so that there re- 

 mains nothing of it but the embryo developed from the 

 essential oosphere, and in this case the cotyledons, or first pair 

 of leaves, are enlarged by the deposit of food-stores so as to 

 fulfil the same purpose ; while in other seeds it is supplemented 

 by an exterior layer of albumen called perisperm. The flower 



