OF POLYPES, 49 



itself to the natural position with the tentacula upwards, and is 

 then rooted permanently by a prominence, which is the mcipi- 

 ent stalk, originating from the under part of the head. Gra- 

 dual elongation of the stalk, afterwards continues to raise the 

 head, and the formation of the zoophyte is perfected."*— So 

 the worm-like embryo of the vesiculiferous Hydroida, a few days 

 after its exclusion from the vesicle, becomes stationary and con- 

 tracts into a circular or spherical spot which always retains its 

 original colour. It is transparent and soft, but in a short time 

 some opaque fleshy spots are visible within it, and are separated 

 by a thin homogeneous transparent substance, which is to form 

 the future polypidoni. " As yet it is exceedingly minute, soft, 

 and gelatinous ; but in the progress of its growth, the soft, thin, 

 homogeneous substance of the exterior becomes more dense, 

 embracing the first formed parts of the fleshy substance, indeed 

 all parts, and the whole jelly, with its thin covering, and con- 

 tinues to advance and to radiate. Then we observe a stem be- 

 ginning to rise from the centre of these radii of roots, which are, 

 in fact, the first formed parts that the little round gemmule 

 shoots out. So that the gemmule is become, not a polypus but 

 a root. It begins then to rise from the centre of the roots, and 

 at length to divide ; so it will at length form on its branches a 

 cell, at the bottom of which cell will gradually be developed a 

 polypus." -f In the Flustra and other ascidian zoophytes the pro- 

 cess is very similar, but in these, instead of the rootlets and little 

 embryo stalk, a cell is the part first formed, in which a polype 

 quickly and almost coetaneously developes itself; and this ori- 

 ginal cell and polypus is as large, as perfect, as fit for every pur- 

 pose to which it is destined as any of those which are in rapid 

 succession evolved from its sides and apex, for age adds only to 

 the number of individuals in the polypidom, and nothing to 

 their perfection. 



* Dalyell in Edin. New Phil. Joiirn. xvii. 412. 

 f Grant, in the Lancet, 1834, Vol. i. p. 229. 



Additional Notes. 



1. BaspaiVs Description of the Structure of Polypes. 



" I have pointed out tliat the polypus is nothing but the continuation of its 



case, which becomes bony or cartilaginous in the lower part, in proportion as 



the upper part is developed. Consequently its tube, or rather its involucrum, 



D 



