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



have proceeded partly from undeveloped tentacles, and have partly arisen independently 

 of these. As far as we are able at present to decipher the difficult physiological signifi- 

 cance of the different organs of sense, we can distinguish four categories of organs of sense in 

 the Medusae, according to the specific energy of each, viz. : — 1. Organs of touch, mechani- 

 cal tools for the perception of touch and pressure ; such, above all, are the tentacles with 

 manifold ectodermal cell formations, which appear specially adapted for the perception of 

 mechanical stimulation — viz., tactile bristles, tactile combs, &c. Besides the tentacles, 

 special tactile organs are often found at many places (namely, at the umbrella margin and 

 the margin of the mouth). 2. Organs of smell or organs of taste, chemical sense tools for 

 the perception of the mixture or rarefaction of the sea-water ; these are probably always 

 present (perhaps hidden under the tentacles already mentioned) ; the clavellae of the 

 Craspedotae, and the funnel-shaped depressions on the rhopalar protective scale of the 

 Acraspedae, may be perhaps regarded as special olfactory organs. 3. Organs of vision. 

 Ocelli or pigment eyes, with or without a lens principally and widely extended on the 

 umbrella margin and the basis of the tentacles ; sometimes adapted for thermatic percep- 

 tions, sometimes for optical (eyes for warmth, eyes for light). 4. Organs for hearing, 

 appearing in several different forms on the umbrella margin, among which we can 

 distinguish two originally different types, velar auditory vesicles with ectodermal otolites 

 and tentacular auditory clubs with endodermal otolites. All the four kinds of organs of 

 sense may be found united in one and the same " sense tentacle," as is the case with the 

 " sense clubs " or " rhopalia " of many Acraspedae. As the umbrella margin is the site of 

 the greater number of different sensillae, in the Medusa? they were usually given the 

 indifferent name of marginal bodies (" corpuscula marginalia"), which only indicated 

 their situation. 



§ 76. Tentacles (t). The tentacles or feeling filaments are by far the most important 

 organs of the umbrella margin of the Medusae, as they not only represent the oldest and 

 simplest organs of sense of this class of urticating animals, but at the same time of their 

 limbs. The tentacles are originally placed on the umbrella margin (" marginal 

 filaments "), and are used as feelers as well as sense organs, also as weapons for attack 

 and defence, as sucking-cups for adhesion by suction (Pectyllidae, Pis. III.-VIIL), as 

 steering organs for swimming, or as manducatory organs for leading the nourishment 

 seized on to the mouth (" filaments of prehension "). Only the small group of the 

 Amaltheida? among the Craspedotae, and the large group of the Ehizostomae among the 

 Acraspedae are distinguished by complete absence of tentacles ; in them the tentacles 

 have undergone retroorade formation and become lost. As the tentacle corona of the 

 Medusae corresponds to that of their ancestors, the polyps, the conditions of formation 

 and structure are in general the same in both classes. The tentacles in most Medusas are 

 placed in a circle on the umbrella margin, just as they are placed on the corresponding 

 peristomial margin or calyx margin in most polyps. In the Medusa? they usually form a 



