512 
Proceedings of the Royal Society 
6. On the Homological Relations of the Coelenterata. By 
Professor Allman, F.R.S.E. 
Abstract. 
In this communication an Actinozoon (Actinia) was compared 
with a Hydrozoon (Hydra), and the various Sub-orders of the Hydro- 
zoa were compared with one another. 
The author agreed with Agassiz in regardingthe radiating cham- 
bers of an Actinia as the homologues of the radiating canals of a 
medusa, but he differed from him as to the true homologies of the 
differentiated stomach-sac of Actinia ; for while Agassiz regards 
this as represented by the proboscis or hypostome of the Hydra 
inverted into its body cavity, Professor Allman maintains that it is 
impossible on this supposition to conceive of the structure of Actinia; 
and on comparing a Hydra with an Actinia , he imagines the tentacle 
to become connate for a greater or less extent with the sides of 
the hypostome and with one another, so that the hypostome of the 
hydra, while retaining its normal position, will thus become the 
stomach of the Actinia, and will at the same time become connected 
with the outer walls by a series of radiating lamellm — the connate 
tentacle walls — separated from one another by radiating chambers, 
the cavities of the tentacles ; while such portions of the tentacles 
of Hydra as still continue free will be represented by a single circle 
of the tentacles of Actinia . 
The author had formerly compared the radiating canals of a 
hydroid medusa to the immersed portions of the tentacles of a 
Hydra , and he still maintains this view. 
The strict parallelism of a siphonophore with a hydroid was 
pointed out, and each of the zooids which combine to form the 
heteromorphic siphonophorous colony was shown — as indeed Hux- 
ley and others had already done — to have its representative in the 
hydroid colony, and to be but a slightly modified form of some 
hydral zooid. 
In order to understand the relations of a discophorous or 
steganophthalmic medusa to the other liydrozoa , he supposes the 
‘ ‘ atrium” of a hydroid medusa, or that part of the main body 
cavity which is still immersed in the solid proximal portion of the 
