REPORT OX THE HYDROIDA. 
li 
Our knowledge of the generative system is imperfect ; but in one family at least tlie 
sexual elements appear to be developed in gonopkores lodged in cavities of the 
corallum . 1 
The Hydroids constructed on the plan thus characterised constitute the fourth sub- 
order, that of the Hydrocorallia. 
As already said there are certain free Medusae (Trachomedusae and Narcomedusae) which 
though they belong to the Craspedotae are known to be developed directly from the egg 
without passing through a polypoid stage. Such a phenomenon, showing as it does the 
omission of a characteristic morphological element in the life-history of the species, renders 
necessary the association of these directly developed Medusae into an independent group, 
which will thus form a fifth suborder of the Hydroida. 
To this suborder the name of Monopsea has been given. It is exactly equivalent to 
the Trachylinae in the system of Haeckel . 2 
If the view which I have elsewhere maintained 3 be correct, that the extinct organisms 
known as Graptolites were Hydroids in which a longitudinal chitinous rod had become 
developed in the walls of the ccenosarc, and in which the hydranths had been replaced 
by sarcostyles, which as we know precede the hydranths in the development of certain 
Plumularidse, in whose early stages sarcostyles alone are present, then the Graptolites 
would represent another primary modification of Hydroid structure. I have accordingly, 
under the name of Rhabdophora, considered the Graptolites as representing one of the 
primary groups of the Hydroida. Having regard, however, to our necessarily very 
imperfect knowledge of these Palaeozoic fossils, I do not desire to assign to this group 
more than a hypothetical value. 
1 See Moseley, Phil. Trans., vol. clxvii., 1877 ; and Report on certain Hydroid, Alcyonarian, and Madreporarian 
Corals, Zool. Chall. Exp., part vii. 
2 In Lirnnocodiurn, whose relations to the Trachomedusae are in many respects so close as to justify us in regarding 
it as an aberrant member of this group, there is some reason for supposing that a Hydroid trophosome shows itself 
in the course of its development. In the tank where this remarkable Medusa first made its appearance a minute 
Coelenterate organism, comparable to a fixed Hydroid destitute of tentacles and otherwise in a condition of extreme simpli- 
fication, was discovered by Mr. A. G. Bourne, and regarded by him as the hydriform trophosome of Lirnnocodiurn. It 
is difficult on any other grounds to explain the association of the two organisms in the same tank ; but as no genetic 
connection between them has as yet been discovered, this must for the present be regarded as hypothetical ; see A. G. 
Bourne in Proc. Boy. Soc., 1884, vol. xxxviii. p. 9. A solitary exception to the generally admitted derivation of the 
Anthomedusae and Leptomedusae from polypoid trophosomes has been recorded by Claparede ( Zeitschr . f. wiss. Zool., 
1861) who believes that he had seen the eggs of an Anthomedusa, referable to the type to which Edward Forbes gave 
the name of Lizzia, directly developed into Medusae while still lying in the walls of the manubrium of the parent. It 
is, however, by no means rare for Medusas to give rise by budding to young Medusae in a way very similar to that in 
which planoblasts are budded off from trophosomes ; such a formation of Medusa buds is well known to occur in the 
manubrium of Lizzia, and so very easy is it to confound the development of these buds in the Medusae with that of an 
egg, that even so excellent an observer as Claparede may well have been deceived by it. But even though Claparede’s 
view of the direct development of the egg in Lizzia be correct, this solitary fact need not be regarded as constituting 
a disturbing element in the classification here adopted. 
3 Gymnoblastic Hydroids, p. 279. 
