THE HYDROZOA. 115 



sac, to one end of which a long filament, often provided with 

 two or three spines near its base, is attached. Many of the 

 Coelenterata, and notably the Physalia, give rise to violent 

 urtication when their tentacles come in contact with the hu- 

 man skin, whence it may be concluded that the nematocysts 

 produce a like injurious effect upon the bodies of those ani- 

 mals which are seized and swallowed by the Polyps and Jelly- 

 fishes. 



As regards the existence of a nervous system in the Hy- 

 drozoa, very diverse opinions have been entertained, and it 

 may be doubted if the problem has even yet received its final 

 solution. I have already discussed Kleinenberg's suggestion 

 that the branched prolongations of the inner ends of the cells 

 of the ectoderm in Hydra, which end in the longitudinal fibres 

 which lie between the ectoderm and the endoderm, may be 

 nerves in their earliest stage of differentiation. Haeckel de- 

 scribes a nervous system in Glossocodon and Carmarina. It 

 consists of a circular band which lies on the inner side of the 

 circular canal of the bell-shaped swimming-organ of these 

 Medusw, and presents a ganglionic enlargement at the base of 

 each of the lithocysts. Of these eight ganglia, the four which 

 correspond to the openings of the four radial canals into the 

 circular canal are the larger. Each of these gives off four 

 branches, one of which follows the course of the radial canal 

 to the central polypite or manubrium ; two others go to the 

 adjacent tentacles, and the last to the lithocyst. 1 



There can be little doubt that the lithocysts, or sacs con- 

 taining mineral particles, which are so frequently found in the 

 Medusae, are of the nature of auditory organs ; while the mass- 

 es of pigment, with imbedded refracting bodies, which often 

 occur associated with the lithocysts, are doubtless rudimentary 

 ej T es. 



The sexual reproductive elements are ova and spermato- 

 zoa — the ova being very often devoid of a vitelline membrane. 

 The fully-formed generative elements lie between the ecto- 

 derm and the endoderm of that part of the body-wall in which 

 they make their appearance. In Hydractinia, as has already 

 been pointed out, the ova appear to be modified cells of the 

 endoderm, and spermatozoa modified cells of the ectoderm ; 



^Haeckel, '_' Beitr,"ge zur Naturgeschichte der Hydromedusen." The ana- 

 tomical disposition of this nervous apparatus accords very well with the recent 

 important observations of Mr. Eomanes on the " Locomotor System of Medu- 

 sae." (** Proceedings of the Koyal Society," December, 1875.) 



