Feb. 1, 1869.] 



HARDWICKE'S SCIENCE-GOSSIP. 



33 



globule of air, •which it is in the act of carrying 

 down as a bubble below the surface of the water 

 (fig. 20). _ 



And this shows us the flexible propodium bent 

 over — as the arm might be bent at the elbow to 

 bring the hand to the shoulder — in the final act of 

 depositing the new vesicle at the basal extremity of 

 the float. It is probably at this moment that the 

 air-bubble receives the albuminous or mucous film 

 from the glands of the metapodium, with which it 

 is found to be invested immediately the propodium 

 is removed and again extended ; but as the coating 

 operation takes place actually within the enfolding 

 foot, it cannot possibly be observed (fig. 21). 



rising independently, she rarely appears to use 

 them. 



The float, which has no vital union with the 

 animal, fulfils the secondary purpose of a safe 

 anchorage for the marsupial sacs which contain the 

 ova and protect the young. It is almost needless 

 to remark that the pouches are not formed by the 

 foot, but are prepared in the nidamental gland, like 

 the egg-sacs of other marine molluscs (fig. 22). 



This figure, drawn from a living specimen, 

 displays one of the semi-transparent ovigerous sac- 

 culi attached by a pedicle to the vesicular float, and 

 needs no explanation. 



The eyes of Ianthina are not visible externally, 

 but are altogether beneath the integuments and in 

 immediate contact with the supra-cesophageal gan- 

 glion. The mouth is retractile from the apex, rolling 

 in like the tentacle of a snail ; it contains two 



Fig. 21. liintldnu exiguu, 

 nat. size. 



Fig. 20. x 12. 



And here is Ianthina exigua with the float 

 complete, with egg-sacs attached to its sides. 



The primary use of the float I have no 

 doubt is to enable the creature to maintain 

 itself at the surface of the water without 

 active effort ; but it can and does remain at 

 the surface when deprived of it, instantly 

 setting to work, however, even under the 

 observer's eye, to repair damages in the 

 manner above shown. When the animal 

 retires within its shell, a large portion of the 

 float is drawn in, completely closing the 

 opening, and is thus analogous, though not 

 homologous, with an operculum. In cases 

 of sudden fright, the float is sometimes nipped 

 off by the violent retractation of the body within 

 the test ; some marine animals — Harpa, for instance 

 —even throw off a portion of the foot itself under 

 similar circumstances. 



The habitat of Ianthina is the deep open sea, 

 where she is found all through the day and night, 

 floating at the surface ; and though probably fur- 

 nished by nature with the means of sinking and 



Fig. 22. Portion of vesicular float with a marsupial pouch 

 attached, containing young of Ianthina exigua x 230. 



brownish horny plates supported on a single pave- 

 ment of cartilage-cells, which seem to be intended 

 to afford a resisting surface to the teeth, an 

 arrangement apparently confined to this isolated 

 genus (fig. 23). 



The tongue is broad, and divided into two lateral 

 portions beset with simple uncinae in a pavimental 

 form without a median series, the individual teeth 



