1896.] BRITISH HXDBOIDS AND MBDTTS^. 471 



measuring iu length the growth of the umbrella. Each series 

 arises either on or very close to the margin, and as the umbrella 

 grows in length so does the distance increase between the margin 

 and each series. 



If all the clusters were present, they would form, at the adult 

 stage, four rows, one above the other — the uppermost set, with 

 six clusters, representing the first stage, and the lowest, with twenty- 

 four, belonging to the adult stage. 



Forbes does not mention the clusters of nematocysts in his 

 description of the species. Qosse has again described the adult, 

 and states that five or six clusters are usually present. Ho gives 

 a figure of a cluster, and of a nematocyst which has discharged its 

 thread. 



Adult Stage. Twenty-four uniform tentacles. — The addition of 

 a fourth set of six tentacles brings the young medusa to its adult 

 stage. The new tentacles develop just like the previous ones, and 

 each one occupies a position midway between the tentacles 

 belonging to the first and third stages. I have only seen one 

 specimen which shows the connexion between the third stage and 

 the adult form. In this specimen the third branch does not leave 

 the main radial canal like the first two branches, but is given off- 

 from the second branch (the branch developed in the third stage) 

 and runs down between the main radial canal and its second branch 

 to the ring-canal, opposite the new bulb. The subsequent growth 

 of the umbrella so separates the branches as to give the appearance 

 of each canal dividing, about the middle of its course, into four 

 branches, each branch running to one of the tentacles placed about 

 equal distances apart on the margin of the umbrella. 



The characteristic features of the different stages may be stated 

 thus : — 



First Stage. — 6 tentacles ; 6 unbranched radial canals ; 6 clusters 



of nematocysts. 

 Second Stage. — 12 tentacles; 1 branch to each canal; 12 clusters 



of nematocysts. 

 Third Stage. — 18 tentacles ; 2 opposite branches to each canal; 



18 clusters of nematocysts. 

 Adult.— 24 tentacles ; 3 branches to each canal ; 24 clusters of 



nematocysts. 



Bach set of tentacles usually appears about the same time and 

 some grow faster than others. Forbes states that some of his 

 specimens possessed only twenty tentacles. 



Abnormal Forms. — First Stage. One specimen with seven 

 radial canals, tentacles and clusters of nematocysts. Valencia, 

 1895. 



Second Stage. One specimen with seven radial canals, each 

 with one branch ; fourteen tentacles and fourteen clusters of nemato- 

 cysts. Plymouth, 1893. 



It is clear beyond all doubt tJiat the hydroid Lar sabellarum 



