REPORT OX THE PENNATULIDA. 49 



REPORT ox THE PENNATULIDA 



COLLECTED IN THE OBAN DREDGING EXCURSION 



OF THE BIRMINGHAM NATURAL HISTORY AND 



MICROSCOPICAL SOCIETY, JULY, 1881. 



BV A. MILNES MARSHALL, M.A., D.SC, PROFESSOR OF ZOOLOGY 

 IN OW^NS COLLEGE, AND W. P. MARSHALL, M.I.G.E. 



( Continual from pafie 86 J 



5. Anatomy of the Zooidii — 



The only important points in which the zonids differ from the 

 polypes are the following : — 



1. They have no tentacles, and no distinct calyx. 



2. They have only two mesenterial filaments, viz., those corres- 

 ponding to the two long filaments of the polypes : like these latter they 

 extend to the bottom of the body-cavity. The remaining six mesen- 

 teries are present, but their free edges below the stomach are not 

 thickened to form mesenterial filaments. 



3. They have no reproductive organs. 



Wliether these distinctions are absolute is, however, very iincertain. . 

 In the younger specimens there appears to be a gradual passage from 

 zooids to polypes (Fig. 6), though whether zooids are in all cases 

 destined ultimately to grow up into polypes must be left for the present 

 undecided. 



Polymorphism, i.e. the existence of structural differences between 

 individuals living together and fundamentally alike, is very widespread, 

 and attains a high degree of development among Hydrozoa, where we 

 commonly find in a single colony (a) nutritive individuals with mouths 

 and tentacles, which digest food not only for themselves, but for the 

 i-est of the colony as well, but are often destitute of means for capturing 

 their prey ; (b) prehensile individuals, I'ichly provided with thread-cells, 

 capturmg the prey and conveying it to the nutritive individuals to be 

 digested, but themselves destitute of mouth or stomach ; {c) 

 reproductive individuals, often with no mouth or stomach. To these 

 may be added, in many cases, loconiotive individuals, whose sole 

 function is to propel the colony through the water; protective indi- 

 viduals, and a variety of other forms. 



Among Actinozoa, on the other hand, though we have an equally 

 marked tendency to the formation of colonies by budding, polymor- 

 phism is exceedingly rare, all the individuals composing the colony 

 being as a rule alike : the most marked example of polymorphism 

 is shown by the group with which we are now dealing — the 

 PennatuUda — and even here we only meet with two kinds of individuals, 

 the polypes and the zooids, between which the distinction may be as 

 in Fiinivuiina bv no means an absolute one. 



