The Origin of Vertebrates

by Walter Holbrook Gaskell

This observation of Gaubert suggests that the place of the lyriform organs in other arachnids is taken in Galeodes by the racquet-organs, and in the scorpions by the pectens. Bertkau, Schimkéwitsch, and Wagner, as quoted by Gaubert, all suggest that the lyriform organs of the arachnids belong to the same group of sense-organs as the porous chordotonal organs of the Insecta, sense-organs which have been found in every group of Insecta, and are generally regarded as auditory organs. Gaubert does not agree with this, and considers the lyriform organs to be concerned with the temperature-sense rather than with audition.

The chordotonal organs of insects have been specially studied by Graber. He divides them into two groups, the poriferous and the non-poriferous, the former being characterized by the presence of pores on the surface arranged in groups or lines. These poriferous chordotonal organs are remarkably constant in position, being found only at the base of the wings on the subcostal ridge, in marked contrast to the other group of chordotonal organs which are found chiefly on the appendages in various regions. The striking character of this fixity of position of these organs and the universality of their presence in the whole group, led Graber to the conclusion that in these poriferous chordotonal organs we are studying a form of auditory apparatus which characterized the ancestor of the insect-group. These organs are always well developed on the hind wings, and in the large group of Diptera the auditory apparatus has usurped the whole of the function of the wing; for the balancers or 'halteres,' as they are called, are the sole representatives of the hind wings, and they are usually considered to be of the nature of auditory organs. It is instructive to find that such an auditory organ serves not only for the purpose of audition, but also as an organ of equilibration; thus Lowne gives the evidence of various observers, and confirms it himself, that removal of the balancers destroys the power of orderly flight in the animal.

A striking peculiarity of these organs in the Insecta, as described by Graber, is the bulging of the porous canal near its termination (Fig. [150], C). This bulging is filled with a homogeneous, highly refractive material, from which, according to Lowne, a chordotonal thread passes, to be connected with a ganglion-cell and nerve. This sphere of refractive material he calls the 'capitellum' of the chordotonal thread. The presence of this material produces in a surface view an appearance as of a halo around the terminal plaque with its central pore; Graber has attempted to represent this by the white area round the central area (in Fig. [150], B). A very similar appearance is presented by the surface view of the flabellum in those parts where the tube runs straight to the surface, so that the refractive material which fills the oval bulging shines through the overlying chitin and appears to surround the terminal plaque with a translucent halo.

Fig. 150 (from Graber).—A, Section of Subcostal Nervure of Hind Wing of Dytiscus to show patch of Poriferous Organs (s.o.). B, Surface View of Poriferous Organs; the White Space round each Organ indicates the deeper lying Refringent Body which fills the bulging of the Canal seen in Transverse Section in C.

Such a peculiarity must have a very definite meaning, and suggests that the canals in the flabellum of Limulus and in the hind wings of insects belong to the same class of organ, the chitinous tubule with its nerve-terminal in the former corresponding to the chordotonal thread in the latter. One wonders whether this sphere of refractive material or 'capitellum' (to use Lowne's phraseology) is so universally present in order to act as a damper upon the vibrations of the chordotonal thread in the one case and of the chitinous tubule in the other, just as the membrana tectoria and the otoliths act in the case of the vertebrate ear.

Patten says that the only organs which seem to him to be comparable with the gustatory porous organs of Limulus are the sense-organs in the extremities of the palps and of the first pair of legs of Galeodes, as described by Gaubert. I imagine that he was thinking only of arachnids, for the comparison of his drawings with those of Graber show what a strong family resemblance exists between the poriferous sense-organs of Limulus and those of the insects. On the course of the terminal nerve-fibres, between the nerve-cell and their entrance into the porous chitinous canal, Graber describes the existence of rods or scolophores. On the course of the terminal fibres in the Limulus organ, between the nerve-cells and their entrance into the porous chitinous canal, Patten describes a spindle-shaped swelling, containing a number of rod-like thickenings among the fibrils in the spindle, which present an appearance reminiscent of the rods described by Graber.

It appears as though a type of sense-organ, characterized by the presence of pores on the surface and a fine chitinous canal which opens at these pores, was largely distributed among the Arthropoda. According to Graber, this kind of organ represents a primitive type of sense-organ, which was probably concerned with audition and equilibration, and he expresses surprise that similar organs have not been discovered among the Crustacea. It is, therefore, a matter of great interest to find that so ancient a type of animal as Limulus, closely allied to the primitive crustacean stock, does possess poriferous sense-organs upon its appendages which are directly comparable with these poriferous chordotonal organs of the Insecta.

The Pectens of Scorpions.

Among special sense-organs such as those with which I am now dealing, the pectens of scorpions and the 'racquet-organs' of Galeodes must, in all probability, be classed. I have given my reasons for this conclusion in my former paper.[^[2]] At present such reasons are based entirely upon the structure of the organs; experimental evidence as to their function is entirely wanting. With respect to the pectens of the scorpion (Fig. [151]), it has been suggested that they are of the nature of copulatory organs, a suggestion which may be dismissed without hesitation, for they are not constructed after the fashion of claspers, but are simply elaborate sense-organs, and, as such, are found equally in male or female. The only observer who has hitherto specially studied the structure of the sense-organs in the pecten is, as far as I know, Gaubert, and he describes their structure together with that of the sense-organs of the racquets of Galeodes, in connection with the lyriform organs of arachnids, as though he recognized a family resemblance between the three sets of organs.