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POPULAR SCIENCE REVIEW. 
I pass on to notice briefly another structural feature, which 
is peculiar to the family, and, indeed, to one section of 
it. Eeference has already been made to the elegant vase-like 
receptacles within which the ova are matured in one of the 
principal divisions of the Hydroida. They are usually distri- 
buted singly over the colony, or massed together on certain 
portions of the stem. In some of the Plumularians, however, 
they are collected in small groups, which are inclosed in a 
curious pod-like case (PI. CX., fig. 4). These corbulce (baskets), 
as they are called, are intercalated amongst the pinnae, and 
in fact take the place of a pinna (PI. CX., fig. 1). They con- 
sist of a mid-rib, from which a number of curved and serrated 
appendages are given off on opposite sides, the extremities of 
which meet above, while the spaces between them are filled in 
by a delicate chitinous expansion, and the whole forms a closed 
receptacle, giving shelter to the reproductive capsules. The 
latter are borne on the mid-rib, near the base of the lateral 
appendages. This well-developed pod-like structure occurs only 
on a limited number of species ; but less highly differentiated 
protective contrivances are met with on other members of the 
family, which clearly represent earlier stages in the process of 
its evolution. By collating these we are able to trace the 
whole course of its history from the first slight modification of 
the normal elements to the perfect corbula, The morphological 
record is complete, and we have the organ before us both in its 
infancy and in its maturity. I may state, to begin with, that 
the corbula is a metamorphosed plumule ; a secondary plume 
modified to form a protective covering for the reproductive cap- 
sules. The mid-rib (PI. CX., fig. 4 a) is homologous with the 
main stem of the plume ; the lateral appendages (PI. CX., fig. 
4 b b) constituting the framework of the case are homologous 
with the pinnae, which, however, have undergone a change 
adapting them to their new office. The calycles are sup- 
pressed, and only the “ calicetti,” sometimes the two lateral 
and sometimes the anterior, remain. These often attain an 
abnormally large size, and give to the structure its crested and 
serrated appearance. In this way the pinna is transformed 
into a support for the chitinous wall of the corbula . 
The course of development that has resulted in the formation 
of this curious receptacle can be clearly traced. I shall give a 
few of the principal modifications which have led up to it. One 
species of Plumularian at least, belonging to the section of the 
family in which the corbula or an equivalent is usually present, 
is altogether destitute of any trace of this organ. Its repro- 
ductive capsules are unprotected as amongst the Hydroida 
generally, and are borne on the main stem or towards the base 
of the pinnae (PI. CX., fig. 3). In a beautiful Australian form, 
