HYDROZOA. 39 



Many zoologists describe the vesicles as "au- 

 ditory organs," the crystals or other bodies which 

 they contain being designated '''otolites." But 

 this view of their nature is altogether hypothetical. 

 Gegenbaur hints that they, perhaps, constitute an 

 apparatus of excretion. The pigment-spots may 

 be regarded with some shade of probability as the 

 earliest indications of organs of sight, which appear 

 amongr the lower forms of the animal kino-dom. 

 In some ocelli, a spherical, highly refractive cor- 

 puscle has been detected by Gregenbaur within the 

 mass of pigment. 



It is by no means certain that a nervous system 

 exists in any of the Hydvozoa. Professor Agassiz 

 describes what he considers as such a system in 

 the nectocalyces of some of the free swimming 

 forms. " In Medusae (he writes) the nervous 

 system consists of a simple cord, of a string of 

 ovate cells, forming a ring around the lower 

 margin of the animal, extending from one eye- 

 speck to the other, following the circular chymi- 

 ferous tube, and also its vertical branches, round 

 the upper portion of Avhich they form another 

 circle. The substance of this nervous system, 

 however, is throughout cellular, and strictly so, 

 and the cells are ovate. There is no appearance 

 in any of its parts of true fibres." But the struc- 

 ture of the tissues here described as nervous is 

 very susceptible of a different interpretation. 

 M'Crady and Fritz Miiller have also speculated 

 on the presence of a nervous system in the Medii- 

 sidce. The former naturalist states that, among 

 several species, he has observed a distinct ganglion 

 in the neighbourhood of each marginal body. 



D 4 



