140 COELENTERATA. 



The zooids are generally attached to the stem in groups called 

 cormidia (Fig. 119). The points of attachment of the cormidia 

 are called nodes, the part of the stem between being internodes ; 

 in such cases the cormidia are said to be ordinate. Sometimes this 

 regular grouping does not occur, and the various zooids bud off 

 separately from the stem ; the cormidia are then said to be irregular 

 or dissolved. 



The nectocalyces by their contractions move the colony through 

 the water; they have a deeply concave muscular sub-umbrella sur- 

 face. The pneumatophore is a hydrostatic apparatus, and, in those 

 forms which have a long spiral stem, serves to keep the body in 

 an upright condition. The gaseous contents is secreted by some 

 of the cells lining the pneumatocyst and can, in some cases, be 

 expelled freely by contraction of the walls of the pneumatophore 

 through one or more openings the stigmata. 



The enteric or gastro vascular system is continuous throughout the 

 colony. The gastral zooids are without oral tentacles, but possess 

 a tentacle arising at their base. This tentacle can be extended to 

 a considerable length and be retracted into a spiral coil. It rarely 

 has a simple form, but, as a rule, it bears a number of unbranched 

 lateral twigs the tentilla, which are also very contractile. These 

 tentacles are invariably beset with a great number of nematocysts, 

 which in many places are closely packed and have a regular arrange- 

 ment. These aggregations of thread-cells are especially found upon 

 the tentilla, where they give rise to large brightly-coloured swellings, 

 the cnidosacs or batteries. 



The gonophores have a velum, a complete gastrovascular system, 

 and a manubrium; but the mouth is nearly always absent. The 

 generative cells are ectodermal, and arise in the manubrium ; they are 

 without radial divisions (as in the Codonidae of the AntJwmedusae). 

 The colonies are generally hermaphrodite, but the gonophores are- 

 male and female. The sexual medusoids frequently become separate 

 from the colony when ripe, but are only rarely liberated as small 

 medusae ( Velellidae), which produce the generative cells during their 

 free life. 



The hydrophyllia are leaf-shaped, and composed of a stiff gelatinous 

 substance ; they are protective in function. All the appendages are 

 developed as buds formed of ectoderm and endoderm, and containing 

 an endoderm-lined cavity which communicates with the cavity of 

 the stem. 



The Siplwnophora are extremely beautiful transparent, marine 



