REPORT ON THE DEEP-SEA MEDUSAE. lvii 



in finer structure from the " velar auditory vesicles " of the Lcptomedusse (§ 83) with 

 which they were formerly usually confounded. 



§ 86. Sense clubs (" rhopalia," or). We designate by this name the peculiar, " composite 

 organs of sense," or " marginal bodies " of the Acraspedse, which are always universal in 

 this section, and only wanting in the lowest and oldest Acraspedse, the Stauromedusse. 

 In place of rhopalia the Stauromcdusee have simple tentacles, and it seems undeniable 

 from their whole structure, situation, and distribution that the rhopalia of the Acraspedse 

 are modified tentacles furnished with several different organs of sense. If we assume 

 that the Tessera, the simplest and oldest among the known Acraspedse, is the common 

 ancestral form of this section (or at least does not differ essentially from the hypothetic 

 ancestral form), the characteristic position of the sense clubs in three higher orders of the 

 Acraspedse is explained as follows : — Of the eight principal tentacles of Tessera the four 

 interradial are transformed into rhopalia in the Peromedusse (Pis. XVIII. , XIX.), and the 

 four perradial in the Cubomedusse (PI. XXVL), whilst the four tentacles alternating with 

 them remain unaltered. In the Discomedusaa, on the other hand, all the eight tentacles of 

 Tessera have become sense clubs ; in fact the majority of the Ephyroniae have four per- 

 radial and four interradial sense clubs (Pis. XXVII.-XXXII.), and it is only in a few 

 genera that their number increases secondarily from twelve to sixteen, rarely from twenty- 

 four to thirty-two (System, pp. 364, 401, 427, 457). As the sense clubs of the Acraspedse 

 in this section have originated independently, and as even the four perradial rhopalia of 

 the Cubomedusse have been formed from " acoustic tentacles " independent of the four 

 interradial sense clubs of the Peromedusse, the former present no homology with the 

 similar auditory clubs or cordyli of the Craspedotae, but only a close analogy ; they are 

 distinguished from the latter by their more composite structure, and also by their pro- 

 tected position in special rhopalar niches (hence Steganophthahnse). The rhopalar niches 

 (" antra rhopalaria," PL XXX. figs. 2-4, on) are ectodermal cavities, which lie in most 

 Acraspedse on the umbrella margin, but which sometimes change their marginal position 

 later on and migrate either on to the dorsal surface of the exumbrella (Cubomedusse, PI. 

 XXVL) or on to the ventral surface of the subumbrella (Drymonema, Pis. XXX., XXXI. ). 

 The sense niches or sense sinuses are enclosed on both sides, usually on their ventral or 

 axial surface, by the paired " sense-folds," the axially projecting medial margins of a pair 

 of sense lobes of the umbrella margin (rhopalar lobes) ; these " plicse rhopalares " (of) are 

 sometimes fused into a plate. On the other hand, the unpaired sense scale or protecting 

 scale (" squama rhopalaris," os), originating from the marginal bit of the exumbrella, which 

 originally formed a connecting bridge between the two sense folds, projects on the dorsal 

 or abaxial side of the rhopalar niche as a protecting roof. In the convex dorsal surface 

 of the protective scale, there is usually a csecal funnel-shaped olfactory depression 

 (" fossula olfactoria," os) whose folded sense-epithelium is furnished with special flagellate 

 cells (olfactory cells). The true sense club, which lies hidden in the niche, corresponds 



(ZOOL. CHALL. EXP. — PABT XII. — 1881.) M k 



