350 ERNEST WARREN. 



(b) measurements of the small nematocysts of the tentacles^ 

 the large nematocysts^ and the specialised nematocysts from the 

 nematophores and ectodermal-sheet lining the hydrotheca. 

 As an indication of the shape of the nematocyst the length- 



breadth index was calculated,, y- x 100. 



Li 



The following points can be noticed : 



(1) A glance at the three columns of the table will show 

 that among both the Gymnoblastica and Calypto- 

 blastica a wide variability occurs^ the length-breadth 

 index ranging from 14-96; but that among the species and 

 genera of well-defined families, e.g. the Eudendriidte, 

 Tubulariida), Sertulariidae, and Campanulariidas 

 there is a distinct tendency for some uniformity. In the 

 four species of the Sertulariida3 the index varies from 22- 

 29, and the mean is 24; and in the six species of the Cam- 

 panulariida3 it varies from 30-42 and the mean is 34. It 

 may thus be said that in certain families, at any rate, the 

 nematocysts have a characteristic shape, which may be ex- 

 pressed by the mean length-breadth index of the various 

 species. As the table stands the family-index for the Sertu- 

 lariidfB is 24 and for the Campanulariidee 34 ; but, of course, 

 a much longer series of species is required before these figures 

 can be in any way accepted. 



(2) Another feature to notice is that when nematocysts of 

 two sizes occur in a hydroid the indices of the two sizes do 

 not, as a rule, diverge from each other very widely, or in other 

 words, the shapes of the large and small nematocysts tend to 

 approach one another. The mean index of the small nemato- 

 cysts, in the case of eight gynmoblastic hydroids, is 66, while 

 the mean index in the same hydroids of the large nematocysts 

 is 69. 



(3) The absolute size of the nematocyst tends to be related 

 to the size of the hydranth. 



In 10 species with a hydranth not exceeding 0"5 mm. in 

 length (mean 0*4 mm.), the mean length of the small nemato- 

 cysts is 4'2 jtt ; and in 12 species with a hydranth exceeding 



