Dermal Sense- Organs of the Crustacea. 301 



slioitly to publish a more detailed account, accompanied by- 

 figures. 



Owing to tiie usually extremely hard chitinous body- 

 covering of the Crustacea, a sensory perception, with the 

 exception of siglit, can only be conveyed by means of struc- 

 tures composed of hairs. In many cases such sensory hairs 

 are externally in no way distinguishable from ordinary hairs 

 and are characterized as sense-organs only by the sense-cells 

 lying beneath their base ; in many instances, however, they 

 have peculiar shapes, and have been described as feathered 

 setaj *, half-featheved setge, cones, knobs, clubs, plugs, threads, 

 styles, cylinders, tubes (" Fiederborsten, Halbfiederborsten, 

 Kegel, Kolben, Keulen, Zapfen, Faden, Griffel, Cylinder, 

 Schlaiiche "), &c. Yet, however different and varied the 

 form of the sensory hairs of the Crustacea, they are never- 

 theless connected together by a continuous series of transitions. 

 The first antenna of the Copepods are of especial interest, 

 since we often find upon them placed close together the 

 greatest variety of sensory hairs with the various intermediate 

 forms. 



At the spot where any kind of capillary structure, it matters 

 not whether a sensory or an ordinary hair, projects from the 

 cuticle, the latter is pierced by a more or less fine pore -canal. 

 The mode of attachment of the hair is of the greatest func- 

 tional importance ; in the majority of cases the capillary 

 structures rest upon a more or less arched, cupola-shaped, 

 chitinous membrane, which rises from the margin of the 

 pore-canal ; this membrane is sometimes soft and thin, so 

 that it gives great mobility to the hair, as is above all charac- 

 teristic of the auditory hairs. The shaft of the hair is gene- 

 rally in two parts, and consists o£ a stouter chitinized. proximal 

 and a paler tliin-walled distal portion, the two being distinctly 

 separated from one another by a slight constriction. 



I. On the Occurrence of Dermal Sense-Organs on 

 THE Bodies op Crustacea. 



In the whole of the Crustacea belonging to the different 

 classes, orders, and families I have discovered sensory hairs 

 on almost all parts of the body. Both the first as well as the 



* Feathered setae are, as is well known, widely distributed among the 

 Crustacea and also occur in the aquatic Dipterous larvte ; I would, how- 

 ever, incidentally remark tliat feathered sette are also found in genuine 

 land-animals, e, y. on the anterior portion (so-called tongue) of the hypo- 

 pharynx of Scutigera, on the palp-shaped appendages of the maxillsa of 

 Lithohius, and on the ptdipalpi of male spiders. 



