GENERAL INTRODUCTION. 
Among the Hydroicls to which the present part of the Report is devoted, the Gymno- 
blastic genera are but sparingly represented, while, on the other hand, a rich and highly 
interesting collection of Calyptoblastic forms has been obtained. These are for the most 
part well preserved — so well, indeed, that in many instances the hydranths and other 
soft parts of the colony have retained their characters in so good a condition as to 
allow of these parts being drawn with most of the natural features of the living animal . 1 
It is however somewhat disappointing to find that a large proportion of species is 
represented by the trophosome alone, a fact probably due to the circumstance that many 
of these species were obtained at seasons when no gonosome is developed by them. 
Notwithstanding, however, the absence of this important element of the colony, it was 
seldom that any difficulty was experienced in assigning a specimen to its true place 
in the system. 
No planoblasts or free-swimming sexual buds of Hy droid colonies are contained in the 
collection placed in my hands. A few deep-sea Craspedote Medusae, some of which are 
probably the planoblasts of unknown Hydroid trophosomes, have formed part of a 
separate Report by Professor Haeckel . 2 
The rare occurrence in the collection of such species as are known to inhabit the 
European Seas is striking, and points to a definiteness in the geographical distribution 
of the Hydroida which could scarcely have been expected, and which certainly contrasts 
with the wide distribution met with among many species of Polyzoa. 
Among the new forms described in the present part of the Report are several which 
render necessary the definition not only of new generic groups, but of new families. 
Among the groups most richly represented in the collection, and of which compara- 
1 Were it not that the Plumularinae have been dealt with in the former part of this Report (Zool. Chall. Exp., 
part xx.), they would form the final group in the arrangement of the Hydroida here followed. The group here indicated 
under the name of Plumularinae is exactly co-extensive with the Plumularidae of the former part. The subordination 
of groups here employed is I believe in accordance with the requirements of Hydroid classification, but had been 
decided on too late to allow of its adoption in the part of the Report already published. 
2 Report on the Deep-Sea Medusae, Zool. Chall. Exp., part vii. 
