TEMPERATURE AND GUSTATORY ORGANS. Ill 



or even if one gently breathes on the gills and under surface of the body, the animal 

 at once becomes greatly agitated. 



The most sensitive areas are the margins of the carapace, and especially the 

 margins of the gill chamber along which the main current of vs^ater passes to the 

 gills, and the anterior surface of the branchial appendages themselves. 



We cannot positively identify the temperature organs. They appear to be 

 short, spike-like projections in which terminates a small tuft of sensory cells. 

 They are distributed over all parts of the carapace and are supplied by the terminal 

 plexus formed by the branching of the haemal nerves. 



They are seen to best advantage in the gills of young Limuli, 2-4 in. long. 

 In those parts of the gills that are most sensitive to heat, i.e., the outer surface of 

 the terminal joints of the exopodites, one may see, in successful methylene blue 

 preparation, a loose subdermal nerve plexus continuous with small clusters of 

 spindle shaped sensory cells. From each cluster a very fine fiber extends outward, 

 through a chitenous tubule, to a short spike situated on the outer surface of the 

 gill. (Fig. 86, A, Ls.) 



Free Nerve-Ends. — In the abdominal appendages that have been injected 

 with methylene blue, large nerve branches may be seen going to the soft integument 

 around the joints of the endopodites. Each branch ends in a group of bipolar 

 or multipolar cells; from them arise many branching fibers that form a rich termi- 

 nal meshwork, lying in or on the ectoderm, but without association with any 

 specialized cells. (Figs. 865, 87.) 



The hyphae of a parasitic fungus sometimes ramify in all directions through 

 or over the surface of the chiten. They usually take on an intensely blue 

 stain, and at first sight might be mistaken for nerve fibers. 



II. Special Cutaneous Sense Organs. 



The Gustatory Organs of Limulus. — Gustatory organs are widely dis- 

 tributed over the neural surface of the head, but they are most highly developed 

 in the appendages that come in frequent contact with the food. In other words, 

 the principal aggregations of these organs are located around the mouth in segment- 

 ally arranged fields. This condition explains their remarkable distribution in verte- 

 brates. There they are primarily arranged in several radiating series on the top of 

 the head, an inconveniently long distance from the present vertebrate mouth, but 

 close to the central areas where the old invertebrate mouth was located. (Fig. 89.) 



The gustatory organs have been most carefully studied in Limulus, and they 

 form the principal basis for my conclusions. They are abundant in the mandibles 

 of the thoracic appendages, except the first and last pairs, and in- the tips of the 

 thoracic appendages. Their presence in the mandibles is indicated by a most 

 beautiful series of reflexes, first described by me in 1892. Organs similar in 



