Rev. T. Hiiicks 07i the History of the Hydroida. 395 



sometimes as many as seven. The sacs containing thread- 

 cells on the umbrella of the planoblast were placed, in the II- 

 fracombe specimens, a little way above the tentacular bulbs ; 

 they are represented in the same position in Alder's figure ; but 

 Allman describes them as extending upwards for some distance 

 from the base of each tentacle. 



I have referred the Goryne ivvplexa of Alder to Gegenbaur's 

 genus ZancJea, and at present I see no reason to change this 

 view. M^Crady, indeed, has instituted the genus Gemmaria 

 for a planoblast which seems to agree in all essential (generic) 

 points with that of our British form ; the trophosome he had 

 not discovered. But I can find no sufficient ground for this 

 addition to an already oppressive nomenclature ; Zanclea and 

 Gemmaria seem to me to embrace one and the same generic 

 type. The main characteristics of Gegenbaur's genus are a 

 bell-shaped umbrella, a moderately long manubrium with 

 simple mouth, four radiating canals, tentacles springing from 

 non-ocellated bulbs and furnished along their course with 

 pedunculated sacs containing thread-cells, and certain pro- 

 minent "ribs" on the umbrella, in which thread-cells are en- 

 closed ; and these are really the essential characters of Gem- 

 maria. Allman, however, has adopted M'Crady's genus, but 

 has not given us his reasons for doing so. It is of course 

 possible that the structure of the pedunculated sacs on the arms 

 of Zanclea costata (Gegenbaur) may differ essentially from that 

 of the similar organs on Coryne im-plexa^ Alder ; but it is 

 hardly probable. It is also possible that the "ribs" on the 

 umbrella of the former may not correspond with the " sacs " 

 on that of the latter, though it seems likely enough that the 

 same kind of structure is intended in both cases. But with 

 our present information, and looking to the whole group of 

 characters, it seems to me better to hold provisionally at least 

 to Gegenbaur's name. 



Alexander Agassiz also accepts M'Crady's genus Gemmaria^ 

 and has given us his reasons ; " the form of the bell," he says, 

 " of the digestive cavity, and of the tentacles is totally dif- 

 ferent in the two genera." " The form of the bell" is a very 

 doubtful generic character ; but the differences in this respect 

 bet^^een the supposed species of Gemmaria are quite as great 

 as those between any one of them and the Zanclea costata. 

 The form of the digestive cavity is a matter of inferior moment ; 

 there are no important difierences in size or structure. The 

 tentacles may not agree in shajye, though there is little dis- 

 agreement in this respect between Zanclea costata and Zanclea 

 implexa ; but they seem to be similar in all essential points. 



On the whole I see no reason for dispossessing the established 

 name. 



28* 



