Fig. 306. Larva of Ascidia mentula. (From Gegenbaur; after Kupffer.) Only the anterior part of the tail is represented.
N´. anterior swelling of neural tube; N. anterior swelling of spinal portion of neural tube; n. hinder part of neural tube; ch. notochord; K. branchial region of alimentary tract; d. œsophageal and gastric region of alimentary tract; O. eye; a. otolith; o. mouth; s. papilla for attachment.
Tunicata. The auditory organ of the Tunicata ([fig. 306]) is placed on the under surface of the anterior vesicle of the brain. It consists of two parts (1) a prominence of the cells of the floor of the brain forming a crista acustica, and (2) an otolith projecting into the cavity of the brain, and attached to the crista by delicate hairs.
The crista acustica is formed of very delicate cylindrical cells, and in its most projecting part is placed a vesicle with clear contents. The otolith is an oval body with its dorsal half pigmented, and its ventral half clear and highly refractive. It is balanced on the highest point of the crista.
The crista acustica would seem to be developed from the cells of the lower part of the front vesicle of the brain. The otolith however is developed from a single cell on the dorsal and right side of the brain. This cell commences to project into the cavity of the brain and its free end becomes pigmented. It gradually grows inwards till it forms a spherical prominence in the cavity of the brain, to the wall of which it is attached by a stalk. At the same time it travels round the right side of the vesicle of the brain (in a way not fully explained) till it reaches the summit of the crista, which has become in the meantime established.
The auditory organ of the simple Ascidians can hardly be brought into relation with that of the other Chordata, and has most probably been evolved within the Tunicate phylum.
Bibliography.
Invertebrata.
(384) V. Hensen. “Studien üb. d. Gehörorgan d. Decapoden.” Zeit. f. wiss. Zool., Vol. XIII. 1863.
(385) O. and R. Hertwig. Das Nervensystem u. d. Sinnesorgane d. Medusen. Leipzig, 1878.