XXVI INTRODUCTION. 



supposed to be auditory ; but the conjecture is hardly 

 borne out by an extended investigation of their structure. 



At the base of the tentacles (Woodcut, fig. xii. ) there 

 is often a collection of pigment-cells (a coloured spot or 

 ocellus) in which a crystalline body is sometimes imbedded, 

 as in Eleutheria and Clavatella. In Tiaropsis diademata, 

 Agassiz describes as many as fourteen highly refractive 

 bodies, or lenses, as forming a crescent within the pigment- 

 spot*. These ocelli are regarded, with much probability, 

 as rudimentary organs of vision, or at least as holding a 

 place in the Hydroid economy analogous to that of the 

 eye in higher organisms. It is interesting to remark that 

 these very simple organs of sense make their appearance 

 only in the zooids which are destined to become free. 



The digestive cavity is lodged in the rnanubrium, and 

 the nutritive material prepared in it passes into the canals 

 and circulates through them ; the oral extremity is some- 

 times lobed, and sometimes furnished with tentacular 

 appendages, which assist in the capture of food. 



The generative elements are developed either between 

 the two membranes that form the walls of the manubrium, 

 or in special sacs, which are borne on the radiating canals 

 (Woodcut, fig. xi. o). They usually occur in the former 

 position amongst the Athecata, and in the latter amongst 

 the Thecaphora ; but the distinction is not universal. The 

 period at which the ovaries and spermaries make their 

 appearance varies considerably; in some cases they are 

 developed before the zooid detaches itself, in others not 

 until long after its liberation. 



The free gonozooid has by no means attained its full 



* Contributions to the N. II. of the U. S. vol. iv. p. 300. 



