1 896.] BRITISH HYDEOIDS AND MBDUS^. 469 



polypites. The reproductive buds are borne in clusters of three or 

 four on the upper portion of the body, and when matured detach 

 themselves as free medusiform zooids (planoblasts) ; they are desti- 

 tute of an ectothecal covering, and are therefore freely exposed to 

 the surrounding water. In an early stage of development the buds 

 are much elongated, and take on their hemispherical form as they 

 approach maturity. The planoblast, at the time of its liberation, is 

 almost hemispherical in form ; the umbrella is perfctly colourless 

 and destitute of thread-cells. The digestive sac or manubrium is 

 very mutable in shape ; normally it is subcylindrical, and some- 

 what swollen at the base, with a slightly lobate mouth. Six radi- 

 ating canals traverse the umbrella, terminating on the margin in 

 as many oval bulbs of a brownish colour, from which six smooth 

 tentacles originate. Both ocelli and lithocysts are wanting ; but 

 lialfway between every two tentacles a minute sac occurs on the 

 margin of the umbrella, containing two or three glittering bodies, 

 which appear to be thread-cells. The planoblast, when detached, 

 bears with it a portion of the peduncle which had formed the bond 

 of connexion between it and the parent stork ; this survives as a 

 somewhat conical process above the base of tlie manubrium, but 

 it is no doubt absorbed after a time. Six is an unusual number for 

 the radiating canals ; amongst the British Hydroida it is met with 

 only in Clavatella (which has also occasionally four) and in the 

 genus Willsia of Forbes." 



I have every reason for believing that the young medusa of Lar 

 is the first stage in the development of the medusa which has 

 been described by Forbes as Willsia slellata. 



At Plymouth in September 1893 the medusoid Willia stellata 

 was fairly abundant, and suflTioient specimens were obtained to trace 

 its connexion with a medusa which exactly corresponds to the de- 

 scription and figures given byHincks of the medusa olLarsahellarum. 

 During April and May, 1895, I again met with Willia stellata iu 

 Valencia Harbour, Irelaud, and was able to confirm the observations 

 made at Plymouth. 



Forbes first discovered Willsia stellata at Oban in 1845, and 

 dedicated the genus to Dr. Will of Erlaugen. Agassiz changed the 

 spelling to Willia. 



Forbes has described the adult form of medusa. The early forms 

 of it may be naturally divided into three distinct stages, according 

 to the number of tentacles : — 



First Stage. Six uniform tentacles. (Plate XVI. fig. 3.) — 

 Umbrella, about one millimetre in length and width, varying slightly 

 in shape but usually subhemispherical, with a broad velum. The 

 manubrium reaches about halfway down the umbrella-cavity. In 

 some specimens taken at Valencia the apical stalk (" Stielcanal " of 

 Haeckel), which connected the medusa to the hydroid, was still 

 visible in the mesogloea of the umbrella, running from the base of 

 the stomach nearly to the ex-umbrella. In most specimens the 

 apical stalk had disappeared. 



