lii THE VOYAGE OF H.M.S. CHALLENGER. 



cells (with palpocilia but without nematocysts) are therefore to be regarded as two differ- 

 ent modifications of cells with tactile bristles (with palpella). The distribution of these 

 tactile cells on the sensitive organs of the umbrella and its appendages varies re- 

 markably. For example, in the Trachomedusae and Narcomedusae special " tactile 

 combs " are found on the umbrella mai-gin, or comb-shaped rows of tactile bristles, tactile 

 rings on the tentacles, and so forth (System, taf. xvii. figs. 9, 10, &c). 



§ 80. Organs of smell (" organa olfactoria"). The peculiar chemical sensilke of the 

 Medusae, which might perhaps be equally or more truly termed organs of taste 

 (" organa gustatoria ") belong to this category. It is easily seen from physiological obser- 

 vations and experiments that the Medusae are very sensitive to change of composition of 

 the salt water, and even to slight rarefaction of it, so that, for example, they sink below 

 as soon as it begins to rain. The organs of chemical perception of sense are not yet 

 known with any certainty, and are probably usually represented by sense cells of the 

 umbrella margin, of the tentacles or of the margin of the mouth. Special organs, which 

 give the impression of sensillae from their situation and composition, may probably also lay 

 claim to this function, such as the marginal clavellae among the Craspedotae and the 

 rhopalar olfactory depressions among the Acraspedae. The " olfactory clubs " or mar- 

 ginal clubs (" clavelli marginales ") are only found in the section of the Craspedotae and 

 there chiefly in the order of the Leptomedusae. In my System, 1879 (pp. 118, 123, 143, 

 taf. viii. figs. 7, 12 ; taf. ix. figs. 3, 8) these clavellae were termed marginal clubs ("cor- 

 dyli marginales") as they are found in those Leptomedusae, in which the auditory 

 vesicles, which they therefore perhaps represent, are wanting (Thaumantidae, Cannotidae). 

 They usually (or always ?) want the characteristic " auditory hairs," which are a distin- 

 guishing feature of the acoustic organs. The pyriform or club-shaped clavellus (PL II. 

 figs. 3, 4, 8), sits with a thin stalk on the umbrella margin, and is, therefore, not to be 

 confounded with the conical disposition of the young tentacles. It contains a caecal, very 

 narrow " canalis clavellaris," which runs out from the coronal canal and is lined with 

 high cylindrical epithelium (PI. II. fig. 8, y). The latter is separated by a thin fulcral 

 plate (z) from the flat epitheluim of the ectoderm (q). The clavellae are found in many 

 Thaumantidae and Cannotidae on the umbrella margin, scattered in large numbers (often 

 several hundreds), between the tentacles, and may, therefore, be regarded as sensillae. 

 This is also applicable to the olfactory depressions (" fossulaa olfactorise, oz) of the 

 Acraspedae. These appear in the Discomedusa? as small, caecal funnel-shaped depressions 

 in the dorsal surface of the rhopalar protective scales (or " funnel plates "), and are 

 Uned with a many folded sense-epithelium, furnished with long flagella (conip. under the 

 rhopalia). 



§ 81. Organs of vision (" organa optica "). Physiological experiments prove easily and 

 with certainty that all Medusae are more or less sensible to the influence of light and 

 warmth. From analogy with other animals we are justified in the conclusion that the 



