HYDKOZOA (HYDROIDS, ETC.). 



In contrast with the Scyphozoa the Hydrozoa lack the 

 inturned oesophagus and the septa dividing the digestive 

 tract, while they have solid tentacles. In size they are on 

 the average much smaller, and colonial forms predominate. 

 In their life-history we frequently meet some wonderful 

 changes, and to describe these we may follow through the 

 life-cycle of the Pennaria, studied in the laboratory work. 



From the egg there hatches out a little oval, free-swim- 

 ming embryo, which soon attaches itself by one end to 

 some submerged rock, while a mouth breaks through at the 

 other, and tentacles grow out around the sides of the body. 

 When a mouth is formed feeding and growth are possible. 

 As the animal grows larger little buds appear on the sides, 

 and these, forming mouths and tentacles, grow into 

 hydranths like the parent. These buds never become free, 

 but the whole colony thus formed has a common digestive 

 tube by which all are connected. On the outside a tubular 

 protecting sheath, the perisarc (p. 299), is developed. After 

 a while buds appear on the sides of the hydranths, and 

 these have a much different history, for they develop into 

 free-swimming jelly fishes. 



These jellyfishes (see Fig. 139) are much like those of 

 the Scyphomedusae (p. 304), being bell-shaped or umbrella- 

 shaped, the mouth being at the end of the handle, while 

 branches of the digestive tract riin to the margin of 

 the umbrella. However, these hydrozoan jellyfishes differ 

 from the Scyphomedusse in the absence of mesenterial fila- 



305 



