The Origin of Vertebrates

by Walter Holbrook Gaskell

The only other kind of sense-organ I have found in connection with these sense-organs are a few spike-like projections, the appearance of which is given in Fig. [149]. I have always seen these in the position given in Fig. [146] (sp.), i.e. at the junction of the surface which contains the sense-organs and the surface which is free from them. They are, so far as I have seen, not very numerous; I have not, however, attempted to examine the whole sense-organ for the purpose of estimating their number and arrangement.

As is seen in Fig. [149], they possess a fine tubule of the same character as that of the neighbouring sense-organs, which apparently terminates at the apex of the projecting spike. They appear to belong to the same group as the other poriferous sense-organs, and are of special interest, because in their appearance they form a link between the latter and the poriferous sense-organs which characterize the pecten of the scorpion (cf. Fig. [152], C).

Fig. 149.—Spike-Organ of Flabellum.

ch.t., chitinous tubule.

Such, then, is the structure of this remarkable sense-organ of the flabellum, as far as I have been able to work it out with the materials at my disposal. It is evident that the flabellar organs, apart from the spike-organs, are of the same kind as those described by Patten on the mandibles and chelæ of Limulus, and therefore it is most probable that the nerve-terminals in the chitinous tubules, and the origin of the latter, are similar in the two sets of organs.

These organs, as Patten has described them, are situated in lines on the spines of the mandibles of the prosomatic locomotor appendages, and are grouped closely together to form a compact sense-organ on the surface of the inner mandible (Lankester's epicoxite) (i.m. in Fig. [155]), so that a surface-view of the organ here gives the characteristic appearance of these poriferous sense-patches. Precisely similar organs are found on the chilaria, which are, in function at all events, simply isolated mandibles, to use Patten's terminology.

On the digging appendage (ectognath), as the comparison of Fig. [155], A and C, shows, the mandibular spines are almost non-existent, and the inner mandible or epicoxite is not present, so that the special sense-organ of this appendage is represented solely by the flabellum.

This sketch of the special sense-organs of Limulus shows that all the appendages of Limulus possess special sense-organs, with the exception of the operculum. All these sense-organs are formed on the same plan, in that they possess a fine chitinous tubule passing through the layers of chitin into the underlying hypodermal and nervous tissues, which terminates on the surface in a pore. The surface of the chitin where these pores are situated is perfectly smooth, although, in the case of the branchial sense-organs, the goblet-shaped masses of chitin, each of which contains a pore, are able to be pressed out beyond the level of the surface.

As to their functions, we unfortunately do not know much that is definite. Patten considers that he has evidence of a gustatory function in the case of the mandibular organs, and suggests also a temperature-sense in the case of some of these organs. The large organ of the flabellum and the branchial organs he has not taken into consideration. The situation of these organs puts the suggestion of any gustatory function, as far as they are concerned, out of the question; and I do not think it probable that such large specialized organs would exist only for the estimation of temperature, when one sees how, in the higher animals, the temperature-nerves and the nerves of common sensation are universally distributed over the body. As already stated, the structure of the branchial organs seems to me to point to organs for estimating varying pressures more than anything else, and I am strongly inclined to look upon the whole set of organs as the derivatives of the lateral sense-organs of annelids, such as are described by Eisig in the Capitellidæ. This is Patten's opinion with respect to the mandibular organs; and from what I have shown, these organs cannot be separated in type of structure from those of the flabellum and the branchial sense-organs.