A COLLECTION OF HYDROIDS. 295 



(3) The loose mesoderm tissue {m.), and the ova (o.) which 

 have already appeared on the ventral aspect, and extend into 

 the base of the chelicera?. 



(4) The chelicerfe {cli.), which are inserted into tlie endo- 

 derm of the hydroid. 



In an older embryo, represented in fig. 18, o.L, clawed 

 cheliceraj (r/.), the oral cone (o.c), and paired, jointed 

 appendages {(ip.) may be seen. 



The larva ultimately breaks out of the gall, and in fig. 19, <>.(/., 

 an empty broken gall is shown. 



The peculiar interest of this example of parasitism is the 

 very definite nature of the response which is made by the host 

 to the stimulation caused by the presence of the developing 

 embryo. Many animal galls and cysts are sim]5le, rounded 

 structures ; but here a structure is formed which is practically 

 an enormous hydrotheca and hydranth, the former l)eing 

 provided with marginal teeth and operculum, and the latter 

 may even possess tentacles. There appears, however, to 

 be no moutli, and very probably the operculum is never 

 opened. 



In this connection it may be noticed that the hydranth of 

 this species exhibits a marked capacity for extended growth. 

 Through some unknown stimulus a hydranth may round itself 

 off and produce, as we have already seen, a short l)ranch or a 

 new hydrotheca, which thus springs from the mouth of the 

 original hydrotheca (fig. 19, &.). 



Ct. Hodge has already desci'ibed a somewhat similar case, 

 where a larval Pycnogonum forms galls on a campanularian 

 hydroid. From a figure given by Karl Semper, in his 'Animal 

 Life,' p. 332, it Avould appear, however, that these galls are 

 simple, irregular structures, and cannot be compared with the 

 definite structure occurring in Sertularella polyzonias. 



(12) Sertularella fusiformis {H lucks). (Text-fig. o, 



C and D). 



Sertularia fusiformis Htiicl-s, 'Ann. Mag'. X.H.." vol. viii. p. 253. 



