A COLLECTION OF JIYDROIDS. 351 



O'o mm. in lengtli (mean l/o nnn.), tlio mean lenu'tli of the 

 nematocysts is 7'2 mm. 



Tlie relationsliip may he ex]n'essed matlieniatically in 

 accordance with Professor Pearson's statistical metliods ; the 

 constants calcnhited from the 22 species are : Mean lengtli 

 of hydi'anth ()'89 mm., mean length of nematocyst 5"9o /,/. 

 Standard deviation of hydranths 0'686 nnn., standard devia- 

 tion of nematocysts l"565,ju. Coefficient of correlation 0-777; 

 probable error of coefficient 0"0-")7. 



It is probable that a similar correlation wonld be Fonnd 

 between the size of ordinary tissne cells and the general size 

 of the hydranth. 



(4) The nematocysts of the nematophores and of the ecto- 

 dermal sheets lining the hydrotheCcG tend to be elongated; the 

 highest index seen is 51, and this belongs to the very aberrant 

 genns Kirch enpaneria. The mean index of the 11 species 

 given in the table is 28. 



In correlation with the development of nematoj)hores and 

 specialised batteries on the sheets of ectoderm, the formation 

 of ordinary nematocysts on the tentacles tends to be weak or 

 almost completely absent. 



EXPLANATION OF PLATES XLY— XLYIII. 



Illnstrating Dr. Ernest Warren's paper " On a Collection of 

 Hydroids, mostly from the Natal Coast." 



Fig. 1. — X 100. Side view of hydranth of Eudendriiim parvuni 

 sp. v., showing the base enveloped l)y a tliin layer of perifcare, terminat- 

 ing at t.p. 



Fig. 2.— X 260. Median longitudinal section of hydrantli of E. 

 parvnm, showing general histology. 



Fig. 3. — x 1100. Upper portion of the basal differentiated ectoderm 

 of hydranth, showing modified terminal cells (cc). 



Fig. 4. — x 75. Male gonophore of E. parvum, with tliree chambers 

 and slight terminal knob. 



Fig. 5. — x 75. Side view of liydi'antli of Endendrinm niigiistum 



