352 THE AUSTRALIAN HYDROMEDUS.E, 



The structure of the male Polyps is in fact the most important 

 characteristic featm'e of our species, and we shall find that this 

 peculiarity is one of greater morphological importance. 



Whilst the ova are borne on irregular verticils of tentacles, the 

 spermatophores are always four in number, and lie radially sym- 

 metrical occupying the same position as in the Meduste (Eucope 

 for instance.) Looked at from above. (Plate VI., fig. 2), the whole 

 has the shape of a Medusa, from the aboral end of the Gastral 

 cavity of the male Polypostyle four canals are given off similar 

 with and homologous to the four radial canals of the Medusa. 



The Polypostyles not only phylogenetically but also ontogene- 

 tically are developed from a Polyp with mouth and arms. After 

 the genital products have commenced to grow the tentacles are 

 retracted and also the mouth obliterates. (Plate V), so that finally 

 the mouthless Polypostyl is the bearer of the large and adult 

 sexual tentacles. 



If this process was arrested, when the tentacles are retracted, 

 and if then the whole of the lower part of the Hydranth would 

 grow out and form a wall between the Radial Canals, which in our 

 case appear as sexual tentacles, then we would have a Medusa 

 before us. 



It appears from this that the Gastral cavity of the Medusa is 

 not homologous to the Proboscis of the Hydranth only, and that 

 the Umbrella is not to be compared to a membrane connecting the 

 oral tentacles of the Hydranth, but rather as a lateral extension 

 of the aboral sexual tentacles. 



The Polypostyls of Laomedea flexmosa and Eucopella campanu- 

 laria which are intermediate between Hydranths and Medusa?, 

 also tend to prove the correctness of the above statement. 



6. EUDENDRIUM PUSILLUM. Nov. sp. 



On Ascidians and other submerged bodies just below low- 

 water mark this Hydroid is extremely abundant in Port Jackson. 



From a creeping Hydrorhiza which covers sometimes very 

 extensive surfaces, stems grow to a height of 12 mm. They are 

 densly and irregularly branched, so that every stem appears 



