THE HYDROMEDUSAE 



body consisting of ectoderm, mesogloea, and endoderm, at one 

 end of which is a mouth, situated on a slight eminence (the 

 hypostoTne) ; through the mouth the internal cavity (coelenteron) 

 communicates with the outer world. Round the mouth are placed 

 tentacles, which are hollow outgrowths of the body, their cavity 

 being part of the coelenteron. 



In the hydroid thus composed the elements of the original 

 diblastula are not far to seek : the primary two layers, ectoderm 



Fifi. 3. 



Fig. 4. 



?,.—}! jldm riridh, attacherl to a piece of weed, o!', ovary ; te, testis. 



4. — Diagram exhibiting the i)lan of structure of liydroids. A, hydroid with wide 

 disc, inanubriuni, and solid tentacles (Tubularian) ; Ji, hydroid with narrow disc and 

 hollow tentacles (Hydra) ; C, transverse section of the body of a hydroid. All the figures 

 show from without inwards ectoderm (strongly hatched), mesogloea (a thick black line), and 

 endodenn (lightly hatched), surrounding the coelenteron. 



and endoderm, and the coelenteron, are still represented. The 

 secretion of a mesogloea, the perforation of a mouth, and the out- 

 growth of tentacles, are the main morphological differences between 

 embryo and adult hydroid. 



Hydroids are either solitary or colonial. The solitary forms, 

 such as Hydra, are capable of reproduction by a process of budding 

 (Braem, 15 ; Seeliger, 16), (Fig. 4, B), in which a part of the body 

 wall, enclosing coelenteric cavity, protrudes laterally ; this elon- 

 gates and forms a mouth and tentacles at its distal end ; the little 

 Hydra, thus produced, becomes constricted off" by an ingrowth of cells, 

 which seal up both its central end and the body wall of the parent. 



