INTRODUCTION. XXXVU 



enclosed by the two membranes (ectoderm and endoderm) ; 

 it is, in fact, a manubrmm without the oral aperture, nutri- 

 tion being provided for by the general circulation. In other 

 cases a membranous envelope (which is the equivalent of the 

 swimming-bell) and rudimentary radiating canals are super- 

 added. In Tubularia a still further advance is made ; the 

 gonozooid, though permanently attached, is furnished with 

 a swimming-bell, in which the canals are present and the 

 orifice, round which are set four tubercles representing the 

 marginal tentacles. (Plate XX. fig. b.} The manubrium 

 is destitute of a mouth. In this form there is every prepa- 

 ration for free existence up to a certain point; but the gono- 

 zooid remains enveloped in the ectotheca, and the swim- 

 ming-bell is converted into a nursery, in which the embryo 

 passes through the later stages of its development. In Gono- 

 thyraa (Woodcut, fig. xiv.) a yet nearer approach is made to 

 the medusiform structure : the umbrella is furnished with 

 tentacular appendages ; and the gonozooid at a certain stage 

 is pushed beyond the orifice of the capsule, and hangs 

 there as if on the very point of escaping and entering upon 

 a separate existence. It maintains its connexion however, 

 and, like the seed-vessel, after ripening and scattering its 

 products it withers away. 



Many other modifications occur ; but those which have 

 been mentioned exhibit the gradual transition from the 

 simplest to the most complex form. 



Occasionally we see the development of the gonozooid 

 arrested at a certain stage, and, instead of becoming free as 

 in normal cases, it continues in connexion with the parent 

 stock. Thus in Syncoryne the sexual zooid is usually 

 locomotive ; but towards the close of the breeding, season, it 

 is sometimes met with in a depauperated condition, without 



