Thread-cells of the Eolidce. 



39 



of the Eolidae contains at its extremity a small ovate vesicle, communi- 

 cating, through the biliary sac, with the digestive system, and opening 

 externally by a minute aperture at the end of the papillae. This vesicle 

 is found crowded with compact masses of thread-cells ; which masses, in 

 Eolis nana, consist of aggregations of small and large thread-cells, iden- 

 tical in size and shape with those of Hydractinia, — on which this Eolis 

 preys, — not contained in capsules, but cemented together by mucus. 

 When we consider that each of the vesicles is in indirect communication 

 with the stomach, I think we may, without presumption, suggest that the 

 masses of thread-cells found in Eolis nana are quasi faecal collections of 

 the thread-cells of Hydractinia, which, protected by their strong coats, 

 have escaped the digestive process. In corroboration of this view, I may 

 mention that the thread- cells of Eolis papillosa, as figured in the work 

 of Alder and Hancock, have a perfect resemblance to those found in the 

 Actinias, which last animals furnish an Abyssinian repast to these car- 

 nivorous mollusca." Dr Wright afterwards found that, as to the above 

 idea, he had been anticipated by his friend Mr Gosse, who, in his 

 " Tenby/' after noticing the existence of the thread-cells in the papillae, 

 remarks : — " The inquiry I suggest would be, How far the presence of 

 thread-cells might be connected with the diet of the mollusc? And 

 whether, seeing the forms of the missile threads vary in different genera 

 of zoophytes, the forms of the corresponding organs in the papillae of the 

 Eolides would vary if the latter were fed exclusively first on one and 

 then on another genus of the former." He afterwards found that Mr 

 Huxley had also doubted, previously to Mr Gosse and himself, whether 

 the thread-cells of the Eolidae were not adventitious. Here were three 

 independent observers to whom the idea has suggested itself ; Mr Huxley 

 had first hinted it ; Mr Gosse suggested it, and how it might be found to 

 be true ; Dr Strethill Wright also had suggested it, and given two in- 

 stances in corroboration of his opinion, and to-night he proceeded to de- 

 tail observations which would, he hoped, entitle it to be enrolled as a 

 proved fact in the records of science. 1st, A specimen of Eolis nana 

 was brought home from Morison's Haven, on a shell covered with Hydrac - 

 tinia, taken from a rock-pool, in which was a profuse growth of Cam- 

 panularia Johnstonii. The papillae of this Eolis contained the two 

 kinds of thread-cells which are found on Hydractinia, together with the 

 large thread-cells which occur within the reproductive capsules of C. 

 Johnstonii. 2d, An Eolis coronata was taken at Queensferry, on a 

 massive specimen of Coryne decipiens, which was very abundant there. 

 The thread-cells of C. decipiens were very distinctive, being very large, 

 oval, and containing a four-barbed dart. The thread-cells of the Eolis 

 and Coryne were carefully compared together, and were found to be 

 identical. 3d, Dr M'Bain and Dr Wright found an Eolis Drummondii 

 on a fine specimen of Tubular ia indivisa. They first carefully examined 



