48 NATURAL HISTORY OF OUR SHORES 



of their history ; the student will find all details in books 

 devoted to the particular branch. 



Naturalists divide the Hydrozoa into two divisions, one 

 of which they term Gymnoblastic (" naked bud "), and the 

 other Calyptoblastic (" concealed bud "). 



In the first the little polypi are always exposed, and in 

 the latter they can withdraw into little glassy cups, often 

 in the shape of miniature wine glasses. Both these forms, 

 and explanatory figures of their alternate generations, are 

 shown in Fig. 13. 



No. 2 in the figure represents a colony of one of the first 

 division (Gymnoblastic). This one is named Syncoryne 

 eximia. 



No. 3 in the figure shows one of its polypi enlarged 

 about twenty times. A is the mouth, B the tentacles, 

 CCA buds developing towards the medusid generation. 

 No. 3 is one of the medusids cast off from the stock. This 

 little medusid bears a name of its own : it is termed Sarsia. 

 (One of the tentacles on the polyp shows stinging cells 

 with the threads extruded.) 



In this species the " division of labour " above men- 

 tioned is performed by different portions of the same 

 polyp, but in most others it is accomplished by separate 

 polypi. 



No. 1 is a tuft of another species, which grows larger than 

 the last : it is Coryne vaginata. In this one the medusid 

 generation is not cast off from the stock, but performs its 

 functions while remaining attached. 



The same figure shows the structure of the Nematocysts, 

 or stinging cells, details of which are given in the explana 

 tion of the figures. 



At extreme low water, and also frequently thrown higher 

 up by storms, may be seen fronds of the great oar-weed, 

 Laminaria diqitata, on which will be seen growing little up- 

 right stems as figured in Fig. 13. They are attached by 



