210 Zoophytes, 



ercise no influence on the growth or development of the mass, except 

 so fat as they are the points through which nutrition is taken in. To 

 separate therefore these parts by so great a physiological separation 

 as life and death, is to do violence to Nature. One plan of life go- 

 verns both, as well in their specific formations as in their mere growth. 

 Many species assume some varieties of general form, depending on 

 the localities in which they grow. The Plumularia cristata, for in- 

 stance, in favourable situations will trail along on sea-weed, and send 

 forth elegant plumes from one to four inches in length ; but near the 

 shore, in less favoured spots, they scarcely reach a quarter of an inch 

 in length, and are exceedingly pale and slight ; sometimes they rise 

 from stones in a single plume, and at others have an irregular and 

 branched appearance ; but, in all, the cells and ovarian vesicles retain 

 their specific differences : but as the polypes and the granular pulp 

 are not in actual contact with these parts at the time in which the 

 specific differences are formed, in w^hat way are these differences so 

 unerringly formed, unless by a power in the sheath itself? The whole 

 of the observations I have been able to make on these creatures can 

 be best explained by supposing a vital relationship existing between 

 the pulp and sheath, and the irritability of the latter can hardly be 

 explained in any other way. It is not more difficult to conceive the 

 vitality of the sheath of the Sertulariadae than of the teeth of ani- 

 mals. Teeth were almost always considered to be endowed with 

 vitality, yet, till the late researches of Nasmyth and Owen, we had no 

 decided proofs of it : their structure alone is now a sufficient proof, 

 but this is still further strengthened by their capability of being in- 

 jected. But though the polypidoms of the Hydroidae have not been 

 injected, nor in them have been detected traces of vascularity or a 

 cellular structure, yet the circumstances under which they exhibit 

 irritability are proofs sufficient to indicate a vital character. That 

 the sheath is not a mere distension of the cuticle of the gemmule 

 every one will allow, since its thickness is nearly alike in all parts, 

 and the growth of the whole is sometimes so rapid that simple ex- 

 pansion would produce a rupture. 



The opinion of Dr. Roget that " polypi are for the most part at- 

 tached to some inorganic shell or base, which may be either of a 

 horny or calcareous nature," seems therefore to be eiToneous : 1st. 

 From the manner in which the growth of the whole, and formation of 

 the cells take place. 2ndly. From the internal alteration of the 

 structure of the sheath, through age. 3rdly. From the shedding of 

 the ovarian vesicles and branches. 4thly. From the irritability of the 



