OBELIA 119 



this condition is arrived at by a somewhat different route from 

 that which leads to the corresponding stage in Amphioxus (com- 

 pared Fig. 13, I X). 



Closely related to Hydra are a large number of marine Coelen- 

 terates, which, from their obviously animal nature combined 

 with their plant-like mode of growth, were known to the older 

 naturalists as zoophytes. One of the most familiar examples of 

 these is Obelia (Fij. 60), which is frequently found attached to 

 rocks or seaweeds near low water mark. 



Obelia differs from Hydra in several interesting particulars. 

 In the first place the asexual process of multiplication by means 

 of budding takes place in a very regular manner, and the buds, 

 instead of separating from the parent, remain connected together 

 to form permanent colonies (Fig. 60, A) in which the constituent 

 individuals or persons (sometimes called zooids) are arranged 

 in a perfectly definite way. The colony is comparable to an 

 arborescent colony of Protozoa such as Zoothamnium or Epi- 

 stylis (compare Fig. 9, n 15), but the individuals of which it 

 is composed are units of a higher order than single cells. 



In the second place the colony develops a common skeleton, 

 secreted by the ectoderm, which takes the form of a slender 

 tube of horny perisarc (ps.) enclosing all the branches and 

 expanding at the end of each into a little cup or hydrotheca (hyth.), 

 occupied by a single zooid. Lastly the colony is polymorphic, 

 the zooids exhibiting a certain amount of differentiation and 

 division of labour amongst themselves. 



A network of root-like branches at the base of the colony 

 creeps over the substratum and serves for attachment. From 

 this network, which is not shown in the illustration, arises a 

 little forest of vertical stems, each of which has a characteristic 

 zig-zag outline (Fig. 60, A). From each angle of the stem a 

 short branch is given off which terminates in a single hydra-like 

 zooid known as a hydranth (hyiL), enclosed in one of the horny 

 cups or hydrothecae, from the mouth of which its tentacles are 

 extended into the water. 



The structure of the hydranth is similar in all essential 

 respects to that of Hydra. In the middle of the ring of tentacles 

 is the mouth, situated on a projecting hypostome and leading 

 into the digestive cavity, and the different hydranths of the 

 colony are all placed in communication with one another by the 

 tubular hydrocaulus, or common stalk (/</c.), enclosed in the horny 



