REPORT ON THE HYDROIDA. 
XXIX 
In every case, with a single recorded exception, when the planoblast referable to the 
type of the Anthomedusae has been traced to its trophosome, this is found to belong to 
the Gymnoblastic section of the Hydroida ; while in every case — also with a single 
recorded exception — the trophosome to which the planoblast of Leptomedusal type has 
been traced belongs to the Calyptoblastic section. 
The exception in the former case is found in Leptoscyphus tenuis, a minute 
Campanularian Hydroid to which I believed myself justified in referring a little Medusa 
found free in the jar in which a specimen of Leptoseyphus was confined, and which, 
though it had not yet attained sexual maturity, was an undoubted Anthomedusa 
referable to the form to which Edward Forbes gave the name of Lizzia . 1 
The exception in the second case occurs in a Tubularian Hydroid described by Claus 
under the name of Ccimpanopsis, the planoblasts of which are referred by him to the form 
known as Octorcliis, which is a Leptomedusa bearing otocysts on the margin of the um- 
brella, and having its gonads or sexual pouches formed in the walls of the radiating canals . 2 
While every planoblast — with the exception of that of Dicoryne — is thus a true Craspe- 
dote Medusa, it is not among all groups of the Craspedotse that planoblasts — confining 
this term to the free sexual buds thrown off from a polypoid trophosome — can be found. 
There are certain Craspedote Medusse (Trachomedusse and Narcomedusse) which, though 
possessing like all the Craspedotse a true Hydroid structure, have not yet been known 
to give rise in the course of their development to a Hydroid trophosome or polyp stage, 
and are probably all developed directly from the egg of the parent Medusa . 3 They are dis- 
tinguished by certain well-marked characters fronTthe other Craspedotse (Anthomedusse and 
Leptomedusse) among which alone we find the planoblasts. The most important of these 
characters consists in the ectodermal otocysts of the Leptomedusse being here replaced by 
peculiarly modified tentacles in the form of short club-shaped appendages which lie 
either free on the umbrella margin or are each enclosed (part of the Tracliomedusse) in 
a special vesicle formed by an ectodermal fold sent off from the epithelial covering of 
the marginal nerve-ring. The}^ have a solid endodermal axis, in certain cells of which 
calcareous concretions (otolites) are formed, while the ectoderm carries stiff “auditory 
bristles.” To these marginal clubs an auditory function has accordingly been attributed. 
Both Trachomedusae and Narcomedusse are also characterised by the comparative 
rigidity of their marginal tentacles, which are originally always provided with a solid 
chorda-like axis instead of being hollow as is almost always the case with the marginal 
tentacles of the Anthomedusse and Leptomedusse. The margin of the umbrella is 
further distinguished by being surrounded by an urticating ring formed by an accumula- 
tion of thread-cells. 
1 Allman, Ann. and Mag. Nat. Hist., May 1864. 2 Claus, Arb. Zool. Inst. Wien, Bd. iv. 
3 Unless the fresh- water Medusa, Linnocodium, should prove an exception ; see A. G. Bourne, Proc. Roy. Soc. Lond., 
vol. xxxviii. p. 9, 1884. 
