REPORT ON THE DEEP-SEA MEDUSAE. 69 



With the naked eye they can he recognised as white granules in the incision between the 

 two ocular marginal lobes. Each rhopalium consists of a conical basal part, the sense 

 knob, of a large sense vesicle on the axial side of the knob, and of a sense fold or 

 protective scale which is placed at the distal end of the sense knob and surrounds the 

 auditory club as well as the eye (comp. PL XVIII. fig. 2, seen from the inside, axial side ; 

 fig. 3, seen from outside, abaxial side ; fig. 4, seen in profile ; fig. 5, seen half from the 

 inside, half in profile). The sense knob corresponds to the basal part of the greatly 

 shortened and thickened tentacle, from which the whole sense club is phylogenetieally 

 derived. It projects between the bases of its two constituent sense lobes, is usually 

 conical in shape, and bears the large spheroidal or oval sense vesicle (" ampulla rhopalaris," 

 oa), a caecal arching outwards of the sense pouch (bo) on its inner or axial side. Just 

 under the ampulla the sense knob is constricted like a neck and surrounded by the large, 

 darkly pigmented sense collar (op). The latter forms an ectodermal swelling, with a thick 

 accumulation of brown or dark pigment and has cmite the shape of a high coat collar, 

 which closes round the neck of the sense knob on the abaxial (external or dorsal) side, 

 whilst it falls obliquely on the axial (inner or ventral) side, and passes into two lateral 

 symmetrically-placed lapels (figs. 2, 3, op). The two lapels of the sense collar are divided 

 from one another by a deep, broad furrow, which is only bridged over below at the distal 

 margin of the collar by a narrow cross bar of pigment. An unpaired axial eye with lens 

 (oc') and pigment cup appears to lie in the depth of the furrow between the end arms 

 and the pear-shaped auditory club (ok), the distal end of the acoustic tentacle rises on a 

 thin stalk immediately below it. The auditory club is white, and consists of an axis of 

 endoderm cells, the last of which forms a large spheroidal otolite sac, closely filled with 

 numerous small prismatic crystals (ol). The ectodermal covering of the auditory club 

 bears auditory hairs which project freely into the niche of the auditory scale (os). The 

 latter forms a protective scale, oval or triangular in shape, arched convexly outwards, 

 concavely inwards, so that it surrounds the auditory club as a protection from the 

 abaxial (external) and distal (lower) side. Two eyes containing a jalaneonvex or 

 biconvex lens in the midst of a cup of brown or black pigment (?) appear placed inside the 

 niche of the scale (on) on the abaxial side of the auditory club (between the otolite sac 

 and the sense collar). All these conditions could, unfortunately, only be indistinctly and 

 incompletely recognised in the poorly-preserved spirit-specimen, so that it was only by 

 aid of comparison with the sense clubs of some other Periphylla that I was able to draw 

 out figures 2-5 reproduced in Plate XVIII. , which can only claim to be approximately 

 or even remotely correct. It may, however, be safely asserted that the sense clubs 

 of Periphylla are modified interradial tentacles, which function simultaneously as 

 acoustic and as optical organs of sense ; in some respects they appear allied more with the 

 sense clubs of Chart/odea, in other respects with those of Nausithoe. In our 

 species there are probably three small eyes furnished with pigment, lens and nerves 



