HYDROID COLONIES. 



697 



shapes, break off and swim away as minute Jelly-fish (Fig. 6, B), dispersing 

 their young as they go. 



Before giving any further account of these reproductive individuals, we 

 must describe the Hydroid colonies a little more closely. They frequently 

 resemble delicately branching trees or bushes, fine moss, or feathers, or 

 sometimes form crusts over stones — hence the name of Zoophytes or animal- 

 plants. Such growths are very common on our coasts, and are still popu- 

 larly considered, on account of their plant-like appearance, to be sea-weeda ; 

 this deceptive appearance being heightened by the fact that the very minute 

 polyps forming the colony are, in many cases, when contracted, entirely 

 hidden within the protective framework. This common protective covering 

 is sometimes of a horny nature and transparent, sometimes hard and chalky, 

 and, round the bodies of the individuals of the colony, may take the form of 

 cups into which the soft polyps can withdraw. The Campanularians received 

 their name from their bell-shaped, protective cups. In the Sea-firs or Sertu- 

 laria, the cups project from each side of the stalk; in the Plumularians, from 

 only one side. In other forms, again, there are no protective cups. The 

 Oaten straw coralline (Tubularia) resembles a number of closely -packed straws, 

 from the ends of which the beautiful little crimson-tentacled polyps project. 



Besides these colonies which invest themselves with hard, tubular cover- 

 ings, there are others in which the chalky parts become so greatly developed 

 as to form massive skeletons so like those of Corals that 

 they were long classed as such. These Hydrocorallia are The 



often found in coral reefs, and would be taken, by the Hydrocorallia. 

 uninitiated, for true coral. In one family, the skeleton 

 of which exactly resembles that of a branched, pink coral, the similarity is 

 even carried out in the structure of the polyp. This family has received 

 the name of the Stylasteridte, because of tlie central style or column which 

 seems to imitate the columella of the true coral. 



Another family of the Hydrocorallia, the Millepora, afibrds an interesting 

 example of the division of the life-work of a colony among its different 

 members. We have already described 

 colonies in which the feeding was 

 undertaken by some of the members 

 and the reproduction by others, but 

 here we have a more remarkable dis- 

 tinction between the different members. 

 If we examine a mass of skeleton of 

 a Millepore, the minute apertures on 

 its surface are often seen to be arranged 

 in groups, one larger, central aperture 

 being surrounded by a number of smaller 

 ones. When the surface of such a 

 skeleton is alive with polyps, the central 

 tube is inhabited by a polyp shorter and 

 thicker than its neighbours, with a mouth 

 and a few knob-like tentacles (Fig. 7, 

 A). From the smaller tubes, long slen- 

 der polyps project, not provided with 

 moutha, but only with tentacles (Fig. 7, 

 B). The outer slender polyps keep up 

 a constant waving motion, bending from time to time over the central poJyp. 



Fig. 7.— Millepora. 



A, Gastric indi\idual. 



B, Tentaculate individual. 



