ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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prised 45 species, of which 16 are new. Among them are several forms 

 whose structure is of great interest, and the results of their investigation 

 will to some extent affect the present views with regard to the systematic 

 relationships of Hydroids. 



A survey of the various families and genera shows a progress from 

 homogeneity to more or less high differentiation. This is most notice- 

 ably expressed in the position of the reproductive elements (gonophores) 

 in the colouy. With Clava as a point of departure, we see in the 

 athecate hydroids which have spindle-shaped hydranths with filiform 

 tentacles a series of stages in differentiation. All the hydranths in a 

 colony of Clava bear gonophores ; there is thus complete homogeneity 

 within the colony. The same is the case with Perigonimus, where the 

 gonophores are found scattered over stems and branches throughout the 

 colony. A somewhat higher stage is seen in Bougainvillia, the gono- 

 phores being congregated on the stems of a few hydranths, while the 

 rest of the colonv is sterile, the earliest commencement of blastostyle 

 formation. In Dicoryne and Eydractinia, the blastostyle formation is 

 complete, as those hydranths which bear the gonophores are more or 

 less atrophied, and are thus no longer used in nutrition. 



From these we come to the thecophorous hydroids, where the unity 

 of the atrophied hydranth and its gonophores is yet more distinctly 

 emphasised by their being covered with a common chitinous sheath. 



Starting from Coryne, and looking at the families 6 that must be 

 derived from it, we seo the same development, but with greater breaks. 

 In this series the differentiation is found to be yet further advanced 

 than in Hydractinia, the blastostyles (atrophied hydranths) being com- 

 pletely transformed, and from representing persons in a colony have 

 become organs in a person. This change is introduced in Myrioiliela, 

 where the blastostyles, which exactly resemble those in Hydractinia, are 

 found on the lower part of the only unatrophied hydranth in the colony. 

 In Tubularia it is found at its highest degree of development, the blasto- 

 styles, which are scarcely recognisable as such, being placed like a circle 

 of raceme-like organs within the hydranth's proximal crown of tentacles, 



The following scheme of derivation is proposed : — 



Corynidse 



Myriothelidae Tubuiariclse 



Clavidae 



Eudendriidae 



riumularida* 



Bou<ramvillida3- 



Haleciidse 



Campanularida) 



Scrtularidfle 



