METRIDIUM MARGINATUM Milne-Edwards 



Material. M. marginatum is quite common in larger 

 tide-pools and below the low-water mark along the Atlantic 

 Coast. Specimens must be stupefied by the addition of magne- 

 sium sulphate in increasing quantity. When reaction to stimuli 

 has been inhibited the solution may be replaced by weak for- 

 malin. Chromic acid is also recommended but has no particular 

 advantage. For microscopic study pieces of stupefied specimens 

 may be preserved in i% osmic acid or in any of the reliable 

 fixing fluids. Every student should receive two specimens and a 

 cross-section through an acontium. 



Descriptive Part 



Metridium marginatum is a representative of that group 

 of the class Anthozoa which is known under the common name 

 of sea-anemones. The majority of Anthozoa are colonial forms 

 and possess a calcareous skeleton. Metridium on the other hand 

 is not a colonial animal and does not possess such a skeleton. 

 Yet its structure is nevertheless typical of an Antho-polyp and 

 like all Anthozoa it has no medusa stage, the polyp itself pro- 

 ducing reproductive cells. 



External features and gastro- vascular system. The body 

 of Metridium marginatum may be divided into a broad foot and a 

 cylindrical column or scapus crowned by a festooned capitulum 

 carrying numerous hollow tentacles. The largest tentacles are 

 nearest the mouth. The oblong mouth is situated at the end 

 of the polyp in the middle of a flat pcristome. In the wall of 

 the column are numerous pores or cmclides which leads into the 

 gastro-vascular space. These pores are always closed and 



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