The Origin of Vertebrates

by Walter Holbrook Gaskell

In our search, then, for the origin of the vertebrate auditory organ in Limulus and its allies, we see so far the following indications:—

1. The auditory organ of the vertebrate is regarded as a special organ belonging to a segmentally arranged set of lateral sense-organs, whose original function was co-ordination and equilibration.

2. Such a set of segmentally arranged lateral sense-organs is found in annelids in connection with the dorsal cirri of the ventral parapodia.

3. If, as has been supposed, there is a genetic connection between (1) and (2) and if, as I suppose, the vertebrates did not arise from the annelids directly, but from a protostracan group, then it follows that the lateral sense-organs, one of which gave rise to the auditory organ, must have been situated on the protostracan appendages.

4. In Limulus, which is the sole surviving representative of the palæostracan group, such special sense-organs are found on both the prosomatic and mesosomatic appendages, and therefore may be expected to give a direct clue to the origin of the vertebrate auditory organ.

5. Both from its position, its size, and its specialization, the flabellum, i.e. an organ corresponding to the flabellum, must be looked upon as more likely to give a direct clue to the origin of the auditory organ than the sense-organs of the branchial appendages, or the so-called gustatory organs of the mandibles.

The Auditory Organs of Arachnids and Insects.

The difficulty of the investigating these organs consists in the fact that so little is known about them in those Arthropoda which live in the water; the only instance of any organ apparently of the nature of an auditory organ, is the pair of so-called auditory sacs at the base of the antennæ in various decapods. We are in a slightly better position when we turn to the land-living arthropods; here the presence of stridulating organs in so many instances carries with it the necessity of an organ for appreciating sound. It has now been shown that such stridulating organs are not confined to the Insecta, but are present also in the scorpion group, and I myself have added to their number by the discovery of a distinct stridulating apparatus in various members of the Phrynidæ. We may then take it for granted that arachnids as well as insects hear. Where is the auditory organ?

Many observers believe that certain surface-organs found universally among the spiders, to which Gaubert has given the name of lyriform organs, are auditory in function. His investigations show that they are universally present on the limbs and pro-meso-sternite of all spiders; that they are present singly, not in groups, on the limbs of Thelyphonus, and that a group of them exists on the second segment of each limb in the members of the Phrynus tribe. In the latter case this organ is the most elaborate of all described by him.

It is especially noticeable that they do not exist in Galeodes or in the scorpions, but in the former special sense-organs are found in the shape of the so-called 'racquet-organs,' on the basal segments of the most posterior pair of appendages, and also, according to Gaubert, on the extremity of the palps and the first pair of feet, while in the latter they occur in the shape of the pectens.