8 ANTIIOZOA IIYDROIDA. 



centric layers, and that the calcareous matter is deposited in 

 true cells/" These observations are intended to support the 

 theory of the independent growth of the polypidom from in- 

 nate living motions or a vegetative principle ; but notwith- 

 standing Link's high authority, and that the more recent 

 observations of Milne Edwards and of Mr. R. Q. Couch may 

 seem to lend it indirectly support, I continue to be of opi- 

 nion that the theory is untenable. No other micrographist 

 has seen, even in the most transparent species, any trace of a 

 vascular system or of a cellular structure ; and, until some 

 organization of the kind can be demonstrated, the polypi- 

 dom is to be reckoned amongst those products which are 

 usually considered by physiologists as extravascular,*}- and 

 owe their origin and form to the soft parts in immediate 

 contact with them. It is very true that so long as this con- 

 tact or relation continues, there is a low degree of life in the 

 extravascular part sufficient to prevent its decay and decom- 

 position, such as is found to be the case in hair, horn, and 

 feathers ; but the growth of the two parts is coetaneous, for, 

 although the expansion of the membrane apparently precedes 

 that of the pulp, it is nevertheless dependent on the growth 

 of the latter for its expansion, and regulated by it. " There 

 is," says Professor Grant, " but one life, and one plan of 

 development in the whole mass ; and this depends not on 

 the polypi, ^vhich are but secondary and often deciduous 

 parts, but on the general fleshy substance of the body." | 



The growth of the polypidom has been accurately observed 

 by many naturalists, and the following seems to be a short 



* Ann. des Sciences Nat. Part. Bot. vol. ii. p. 321.— Link himself appears to 

 admit that there was some inaccuracy in his first observations. See Reports on 

 Zoology and Botany, published by the Ray Society, p. 474. 



f By extravascular we mean a part not permeated by nutritious or absorbent ves- 

 sels, and undergoing no interstitial change when once formed. M. S. L. Loven, 

 treating of the polypidom, says, "This once produced, is but an inanimate excre- 

 tion, from every part of which the living portion which made it becomes detached, 

 and does not further nourish it. It is within this protecting envelope that the polyp 

 is developed ; that it takes the number, form, and dimensions of all its parts ; it then 

 breaks mcclumically through its capsule, and is arrested in its growth." — Micros. 

 Journ. i. p. 106. 



X Outlines of Comp. Anatomy, p. 14. 



