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(gonocheme) or indirectly (blastocheme), to sexual maturity, 

 and produces ova or spermatozoa. 



" 2. The ova of the medusiform bud undergo, like those of 

 the sporosac, a continuous development, by which they 

 become transformed into hydriform trophosomes, while these 

 trophosomes .ultimately give origin, by buds, to medusse 

 identical with those from whose ova the trophosome was 

 directly developed." 



The fifth chapter is devoted to the histology of thehydroida, 

 and in it will be found much of the greatest interest to the 

 microscopical observer, who cannot, when the opportunities 

 of observing these animals, and more especially the medusi- 

 form gonophores, offer themselves, employ his instrument to 

 greater advantage than in the study of their minute structure, 

 several points in which appear to us, notwithstanding what 

 has been done by so many able observers, still to demand 

 further investigation, more particularly with high powers. 



In the case of the mrdusae the transparency of the tissues, 

 while alive, is so great as to afford the utmost facilities to the 

 skilful manipulator, and they present a further advantage in 

 their persistent vitality, which renders it possible to observe 

 the structure, for instance, of living muscular fibre, inider 

 conditions of compression and disintegration, which in almost 

 all other cases would deprive it of vitality ; and the import- 

 ance of being able to examine tissues in the living condition 

 is becoming daily more and more obvious. 



The histological points, as it appears to us, which more 

 especially require further investigation, are: 1. The real 

 structure of the gelatiniform substance which makes up so 

 much of the thickness of the disc. 2. The real conformation 

 of the muscular tissue. S. The still more important question 

 as to the existence or not of a nervous system. 



1. With respect to the first of these points Professor Allman 

 says, that he has failed in his attempts to detect any structure 

 in the gelatinous portion of the umbrella in the naked-eyed 

 medusae ; and it must be acknowledged that, speaking gene- 

 rally, no obvious structure can be detected in this substance in 

 that class. Nevertheless, as analogy would lead us to 

 suppose, that it must resemble the same tissue in the 

 Steganophthalmata, in which, as long since shown by 

 Schultze, a sort of cellular structure is discernible ; and also 

 since it was shown by Mr. Busk,^ in 1842, in Turris tieglecta, 

 that the gelatiniform substance, at any rate, at the base and 

 walls of the stomach, is distinctly cellular, it is a question 

 well worthy of investigation, whether an analogous condition 



' '- Traus. Mic. Soc. Loud./ lSi2. Vol. III. 



