480 Mr. G. H. Grosvenor. [Nov. 3, 



of discharged nematocysts, used by their original owners in 

 defence against the devouring Solids 1 



1. We have seen that M. Hecht considered the immunity of Solids 

 towards the nematocysts of Co3lenterates to be in some way due to 

 their possession of nematocysts of their own. But, of course, when 

 the origin of the nematocysts of Solids is known, it is obvious that 

 this immunity is not a result, but a necessary condition of their 

 possession. 



Boutan (7) contends that the immunity of ^Eolids is not absolute, 

 and that it is due at any rate in part to the secretion of mucus and 

 the mode of attack. He dropped a healthy d$. papillosa into the 

 middle of the tentacles of an expanded actinian ; the ^Eolid secreted 

 large quantities of mucus, and after 'a short struggle escaped, and 

 eventually devoured the actinian. Another individual of the same 

 species, which had just been depositing ova and had so exhausted its 

 supply of mucus, being treated in the same way as the last, was 

 itself devoured. A third specimen, which had also been depositing 

 ova, was simply placed in the same vessel with an actinian, which it 

 devoured, beginning at the base of the column. 



JE. papillosa is, of course, an actinian-eating species and so accus- 

 tomed to dealing with these animals. I have found Spurilla neapolitana 

 to be equally capable of escaping from the clutches of an anemone. 

 On the other hand, a particularly large and active specimen (6 cm. 

 long) of R. peregrina, a species which usually feeds on hydroids, on 

 being dropped among the tentacles of an expanded actinian, started 

 to crawl out, but when the actinian began to envelope it with tentacles, 

 it drew back and writhed in the centre, secreting mucus copiously 

 and throwing off all its cerata. After several more vain attempts at 

 escape it remained almost motionless, and would undoubtedly have 

 been devoured had I not removed it with a pipette. The mucus was 

 found to be crowded with the discharged nematocysts of the anemone 

 and a few from the cnidosacs of the Bizzolia. The cast-off cerata were 

 also pierced with the barbed threads of many actinian nematocysts. 

 As for the -^olid itself, its tentacles and rhinophores, the anterior 

 processes of the foot and the whole posterior part of the same, seemed 

 quite paralysed, and remained stiff and useless for 3 or 4 days. 



These experiments show not only that the immunity is by no means 

 complete, but also that it depends on something besides a copious 

 secretion of mucus. 



2. The presence of only undischarged nematocysts in the cnidosacs 

 of ^Eolids may be due to the nematocysts never having been discharged, 

 or to the threads of discharged nematocysts having re-inverted by the 

 action of the digestive fluids. 



When a hydroid polyp is plucked off and examined in sea water the 



