THE HYDROZOA. 21 



any Gregarina, Rhizopod, or Radiolarian ; and neither descrip- 

 tion nor definition of the corresponding classes, of a thoroughly 

 satisfactory kind, is attainable. 



No such difficulties beset us in studying the next class, which 

 embraces the Hydroid polypes and the Meduste, and which may 

 be defined with as much precision as any group in the Animal 

 Kingdom. 



VI. THE HYDROZOA. 



All the Hydrozoa exhibit a definite histological structure, 

 their tissues primarily presenting that kind of organization 

 which has been called cellular. Again, the body always ex- 

 hibits a separation into at least two distinct layers of tissue 

 an outer and an inner which have been termed, respectively, 

 ectoderm and endoderm. The endoderm is that layer which 

 lines the inner cavities of the body, from the mouth inwards ; 

 the ectoderm is that which forms its external covering. 



These two layers are shown in the accompanying diagram- 

 matic sections of the leading forms of Hydrozoa, the ectoderm 

 being represented by the thin line with the adjacent clear space, 

 the endoderm by the thick dark line (Fig. 6). 



A third distinctive character of the Hydrozoa is, that the 

 digestive cavity communicates directly, by a wide aperture, with 

 the general cavity of the body; the one, in fact, passing by 

 direct continuity into the other. Furthermore, the digestive 

 sac is not in any way included in the substance of the rest of 

 the body, but stands out independently, so that the outer wall 

 of the digestive cavity is in direct contact with the water in 

 which the animal lives, and there is no perivisceral chamber. 

 The like is true of the reproductive organs, which may vary 

 very much in form, but have the common peculiarity of being 

 developed as outward processes of the body wall, so that their 

 external surfaces are directly in contact with the surrounding 

 medium. 



The majority of these animals seize their prey by means of 

 tentacula developed either around the mouth, or from the walls 

 of the digestive cavity, or from the body wall ; and these tenta- 



