gonactinia prolifera in firth of clyde. 29 



Characteristics. 



Gonactinia is one of the primitive anemones, resembling in 

 structure Edwardsia, which is now generally considered to be 

 the simplest and most generalised type of Actinian which exists 

 at the present day. 



Gonactinia is a small, reddish anemone, about 2-3 mm. in 

 length and 1-2 mm. in breadth. It has sixteen non-retractile 

 tentacles, in two rows, and also sixteen mesenteries, of which 

 eight are macromesenteries. Four of these latter bear gonads. 



Gonactinia has the power, quite exceptional among Actinians, 

 of reproduction by transverse division. A circlet of tentacles 

 buds out about the middle of the column, and a constriction is 

 formed just above. The anemone is then divided by this con- 

 striction into two segments, and both of these segments form 

 new individuals. The lower segment developes a stomodaeum 

 only after it has become separate from the upper. The whole 

 process resembles the stabilisation of the scyphistoma among 

 the Discomedusae. 



Gonactinia sometimes exhibits lateral budding (4), or even 

 longitudinal division (5), but the latter process is abnormal. 

 Sexual reproduction is also found in Gonactinia. 



Some have considered Gonactinia to be merely the young of 

 such a form as Anthea cereus, but such a view is controverted 

 by the fact that the ripe sexual elements have been observed. 



Gonactinia has little power of adhesion, and is easily detached 

 from its point of support. Prouho (5) has observed that it has 

 the power of swimming. It swims head foremost, by means of 

 bending and stretching its tentacles in rapid rhythm. Sars (1) 

 notes another mode of progression. It can move along after the 

 manner of a " looper " caterpillar. Bending over, it takes hold 

 of the surface of attachment with its tentacles:, and shifts its foot 

 close up to the tentacles ; then it fixes its tentacles anew, further 

 over, and again drags its foot along. In this way it is able to 

 crawl in any direction. The original description of Gonactinia 

 prolifera by Sars (1), which is not readily accessible, is trans- 

 lated by Blochmann and Hilger in their paper on Gonactinia (4). 

 They furnish also a specific diagnosis of their own. 



