XIV I N TRODUCTION . 



and stretching vertically through the cavity inclosed by 

 the column. Each principal septum (Plate XI. fig. 1, a), 

 in any of the normal species, is inserted, by its outer edge, 

 into the column-wall throughout its entire height ; by its 

 lower edge, into the base, from the wall to the centre ; by 

 its upper edge, into the disk, from the margin to the mouth ; 

 and, by its inner edge, into the stomach, from the lip, almost 

 to the free bottom of that viscus. From thence the inner edge 

 recedes with an arching outline, and is free, until it is 

 gradually merged in the lower edge at the centre of the 

 base. Between these primary septa, others are developed 

 in succession, partitioning off the imperfect chambers thus 

 formed. But the septa of each successive cycle, while still 

 inserted in the column-wall throughout, spring from the 

 stomach at higher and higher points, and terminate at 

 points more and more remote from the centre of the base. 

 The number of septa depends, to a certain limit, on the age 

 of the individual, but in Peach la it never exceeds twelve, 

 and in Halcampa microps, eight. 



In Peachia, the tissue of the septa is very dense, and 

 still more so in T. crassicornis, where it assumes a firmness 

 almost cartilaginous, and a decided blue colour. 



The muscular tissue of the disk protrudes in the form 

 of hollow cones, which are the tentacles : each of these 

 springs from an interseptal chamber, and hence their deve- 

 lopment is in cycles corresponding to that of the septa. 

 The fibres which compose their walls are very delicate. 



3. Nervous and Sensory System. I have been as unsuc- 

 cessful as my predecessors, in my search for nervous threads 

 or ganglia ; still, I have little doubt that such exist. I 

 should expect their presence in the form of a ring, sur- 

 rounding the mouth, perhaps with a pair of ganglia at the 

 gonidial tubercles, distributing threads to the tentacles. 

 I have never observed any trace of auditory vesicles or 

 otolithes, nor any organs that I could regard as eyes ; not 

 even in the rudimentary form of those aggregations of pig- 

 ment-cells, that occur on the margin of the Naked-eyed 

 Medusa?. A delicate sense of touch certainly exists, dis- 

 tributed over the entire surface, but specially localized in 

 the lips and the tentacles. The occasional elongation of 

 one or more of these latter organs, and their employment 

 (as described at pp. 34 — 36. infra), indicate the existence 

 of an active tactile faculty, and not merely of passive 



