304 'Journal of Comparative Neurology and Psychology. 



2. Organs for the Reception of Chemical Stimuli. — There has 

 been considerabl e difference of opinion as to the structure and 

 nerve supply of the setae in Crustacea, and a still greater differ- 

 ence of opinion as to their function. The confusion is intensified 

 by the fact that many investigators generalize about "sensory 

 hairs" w^ithout specifying just what hairs they mean, some 

 contrast them with "ordinary hairs" others distinguish ordi- 

 nary hairs, touch hairs, taste hairs, olfactory bristles, and 

 auditory hairs, while still others contend that all hairs are sensory 

 and reduce them to two types, tactile and chemical. A brief 

 resume of the more important of these views will be in place here. 



The first careful work on the sensory hairs of Crustacea was 

 that of Leydig ^ in which he called attention to modified setae on 

 the distal half of the outer antennules and to these setae he gave 

 the name of olfactory tubes. Their walls are described as being 

 thick at the base and growing thinner, more delicate at the end. 

 Above the basal portion there is a slight nick in the walls, and 

 above this the tube enlarges and, tapering somewhat toward a 

 point, ends in a sort of button, which is either covered with a thin 

 membrane or is perforated. Near these bunches of olfactory 

 hairs there are feathered bristles of the ordinary heavy kind, and 

 also lighter feathered bristles or "touch" hairs. The inner 

 antennules and the antennae present the same appearance except 

 that the olfactory setae are lacking. In a later work- Leydig 

 examined the antennules of Cambarus pellucidus, the blind 

 crayfish, and found that the outer ones were composed of about 

 36 segments, and that olfactory setae occurred from the 15th seg- 

 ment to the end. 



On the basis of this Wright^ claimed that C. pellucidus is 

 better supplied with olfactory setae than the crayfish with eyes, 

 since C. propinquus, the most closely allied species, has only 

 18 or 19 segments, and only the distal 9 of these bear olfactory 

 setae. Faxon,^ however, found C. propinquus may have as 



* Leydig, F. Ueber Geruchs- und Gehororgane der Krebse und Insekten. Miiller^s Archiv jiir 

 Anatomie und Physiologie, Jahrgang i860, pp. 265-314, i860. 



^ Untersuchungen zur Anatomie und Histologie der Thiere. Bonn. 1883. 



^Wright, R. R. Comparison of the So-called Olfactory Organs of Cambarus pellucidus with those 

 of Cambarus propinquus. American Naturalist, Vol. 18, pp. 272-273. 1884. 



■* Faxton W. Revision of the Astacidse. Memoirs of the Museum of Comparative Zoology, Harvard 

 College, Vol. 10, Part 4, pp. i-i86. 1885. 



