392 Rev. T. Hincks on the History of the Hydroida. 



between the two being that in Cladonema the thread-cells are 

 distributed in several clusters (about five) along the arm, while 

 in Clavatella they are gathered into a single spherule at the 

 extremity. Van Beneden has observed that in an early stage 

 the tentacle of the planoblast is destitute of branches, and is 

 furnished with two suckers : it appears that in a yet earlier 

 stage it has only one. Allman, in the concluding part of his 

 great work on the Tubularian Hydroids, has made the same ob- 

 servation, and he adds that "in a very early stage" (before the 

 liberation of the gonozooid) " the marginal tentacles are quite 

 simple^ At the time of detachment one suctorial appendage 

 has been developed ; and, according to the observation of Van 

 Beneden, a second makes its appearance before the branching 

 of the tentacle commences*. 



Judging from Allman's exquisite figure, the bifurcation of 

 the tentacles begins at a later period in Cladonema than it does 

 in CJavatella ; in the latter the minute lobes on the margin of 

 the disk, in which they originate, exhibit almost immediately 

 a slight depression in the centre, indicating the future course 

 of development. 



I was much interested in watching the curious habits of the 

 young Cladonema. It was exceptionally hardy, and throve 

 well in confinement. But after a short time it proceeded to 

 reverse its swimming-bell (just as a man might throw off" his 

 coat on commencing a piece of hard labour) , and, firmly planting 

 itself on its suctorial appendages, made a vigorous attack on 

 the minute crustaceans that swarmed in the surrounding water. 

 The manubrium, unimpeded by the restraint of the umbrella, 

 and placed on a decided vantage ground, moved eagerly from 

 side to side, and with the aid of its well-armed oral lobes 

 succeeded, I have no doubt, in securing abundant supplies. 

 The zooid in this condition presented an extraordinary figure : 

 the characteristic grace, with much of the familiar appearance, 

 of the Medusa had vanished; and the stout cylindrical proboscis, 

 mounted on a kind of pedestal and swaying to and fro as the 

 little Entomostraca played about it, offered a strange contrast 

 to the exquisite form of which Van Beneden could say, " rien 

 n'est gracieux comme un Cladon^me." 



The retroversion of the umbrella has been noticed in several 

 species and by several observers — but always as occm'ring late 

 in the life of the zooid, and shortly before the escape of the 

 generative products. When the walls of the manubrium were 

 already laden \yii\\ ova, I have seen the swimming-bell in the 

 planoblast of Syncoryne eximia thrown back and contracted 

 into a small mass, to which the tentacles were still attached. 

 * ' La Faune Littorale de Belgique : Polypes,' 1866, p. 142. 



