SLIME BUDS. 119 



tion of mucous cells and hair cells in one organ is comparable with the association 

 of sensory and mucous cells' in the slime buds of arachnids and in the olfactory 

 organs of insects (Necrophorus, Dahlgren and Kepner). If the short, rod-like 

 bodies in Limulus are true sensory cells, then the morphological resemblance 

 between an arachnid slime bud and a vertebrate neuromast is very striking. 

 (Figs. 8s, D-H.) 



According to Maurer, there are some cases in the vertebrates where the 

 lateral line organs still remain in a condition approaching that in the arachnids, 

 for he regards the slime buds in Myxine and Bdellostoma as probably representing 

 modified lateral line organs of Petromyzon. In other words, in Myxine and 

 Bdellostoma, the mucous sacs are sense buds, in which all or nearly all the cells 

 secrete mucous. 



In the vertebrates, however, the secreting function is usually relegated to 

 separate cells in the adjacent ectoderm, the "supporting" cells apparently re- 

 taining their secreting function only in exceptional cases (Maurer). In reply 

 to an inquiry on this point, Prof. C. Judson Herrick writes me that "the line 

 organs of vertebrates are so exceedingly variable that I would not venture to 

 generalize, with my present knowledge, on the relation between the sensory and 

 the mucous cells; but certainly in some cases, and I think as a rule, they are closely 

 associated. The mucous cells are I think generally absent in the non-sensory 

 parts of the lining of the canals. As to the function of the mucus, I have hitherto 

 regarded it as like the mucus of the general body surfaces, protective. But 

 in view of Parker's work on the function of the lateral canal sense organs as re- 

 ceptors for slow vibrations, it may be that the mucus and the cilia of the hair 

 cells both enter into the formation of the cupula which overlies the lateral line 

 organs much as in the case of ampulke of the internal ear and that the whole cupula 

 assists in the stimulus of the sensory cells." 



However, it is clear that we must go farther back than primitive vertebrates 

 for our explanation. In Limulus, for example, we have the same kind of gland 

 cells intimately associated with cutaneous sense organs, and it is extremely improb- 

 able that the abundant mucus there serves either to protect an already practically 

 impervious covering, or to assist, by slow vibrations, in the stimulation of the 

 sensory cells. 



6. The function of the slime buds in Limulus is not apparent. The presence 

 of the mucoid secretion is obvious enough in both vertebrates and arachnids, but 

 a satisfactory explanation of its purpose is not available; and it is difficult to 

 account for the rich innervation of these organs in Limulus, or for the presence of 

 sensory or nerve cells in them, or for their association with other sense organs, 

 on the ground that they are mucous glands and nothing more. 



The only conclusion open to us at present is that first suggested by me in 1889, 

 namely that in the arachnids and primitive vertebrates, the mucous secretion serves 

 to absorb certain chemical substances held in solution, and to thus intensify 

 their action on the nerve ends. This explanation would account for the abund- 



