392 EVENINGS AT THE MICROSCOPE 



predominal instinct. On both these points, however, light 

 has been shed. 



This larger Sabella tube was not the only one infested 

 with the parasites. I observed them on at least two smaller 

 specimens of the same species, in the same situation, and 

 with precisely the same movements. Th& extremity of 

 one of these smaller tubes I cut wholly off, and placed 

 in the live-box of the microscope. Two of the parasites 

 only were on it, which were active at first, but in about 

 an hour probably from the exhaustion of the oxygen in 

 the small quantity of water enclosed they decomposed, 

 or rather disintegrated, the outline dissolving, and the 

 external cells becoming loose and ragged, and the whole 

 animal losing its definite form. 



One of these specimens, however, while yet alive and 

 active, afforded me an observation of value. I had already 

 associated the form conjecturally with the Hydroid Poly- 

 pes, and was inclined to place it in the family Corynidce^ 

 considering the arms to be tentacles, and the head-lobe 

 to be homologous with them in character, but abnormal 

 in form. It appeared to be a three-tentacled Coryne, with 

 the tentacles simple instead of capitate. But while I was 

 observing the individual in question, I saw it suddenly 

 open the head-lobe, and unfold it into the form of a broad 

 shovel-shaped expanded disk, not however flat, but with 

 the two halves inclining toward each other, like two leaves 

 of a half -opened book. This immediately reminded me of 

 the great sucking-disk which, as I lately told you, I had 

 seen evolved from the obtuse summit of Stauridia producta, 

 and confirmed my suggestion of the natural affinities of 

 the form. 



Altogether unlike, in their shape, and in the unwonted 



