The Sense-Organs connected with the nervous system are few and simple. There are sensory cells in the ectoderm, on the margin of the velum, on the velar tentacles, and especially in clumps on papillae of the cirri around the mouth, which are probably tactile. In the roof of the oral hood there is a sensory structure, the "groove of Hatschek," which is supposed to be an organ of taste. The olfactory pit alluded to above opens externally on the left-hand side of the snout. It is ciliated internally and leads to the so-called olfactory lobe, an antero-dorsal hollow outgrowth from the brain. In the young animal the olfactory pit opens by the neuropore into the central canal (Fig. 80, A), but that passage is closed in the adult. Possibly the olfactory pit is homologous with the hypophysis or pituitary body of Vertebrates, the homologue of which in Tunicata has a ciliated funnel. Finally, the median cerebral eye (Figs. 80 and 81) is a mere pigment spot in the anterior wall of the cerebral vesicle, and a series of somewhat similar pigment spots occurs along the floor of the central canal in the spinal cord.[[114]] There is no known auditory organ. On the under surface of the oral hood patches of ciliated epithelium drawn out into rounded lobes were called by Johannes Müller the "Räder-organ." This is probably of use in drawing water inwards to the pharynx, but it may also be a sense-organ.
Fig. 81.—Branchiostoma lanceolatum. Anterior portion of central nervous system from above, showing dorsal and ventral spinal nerves. (From Willey, after Schneider.)
The Gonads are segmentally arranged along the sides of the body, projecting into the atrial cavity at the sides of the pharynx and intestine. In some species the gonads are paired, but in others belonging to the genus Asymmetron (p. 137) only a single series, that of the right side, is present. In the common Amphioxus (Branchiostoma lanceolatum) there are about 26 pairs (Fig. 70, B), lying in somites 25 to 51; and ovaries and testes are found in separate individuals in all other respects. Each gonad is surrounded by a layer of coelomic epithelium. The gonad must therefore be regarded as having grown down from a myotome of the body-wall into a coelomic pouch, carrying before it the coelomic and then the atrial epithelium (Figs. 72, and 74, A, g). Eventually the gonads, when ripe, burst through the layers of epithelium, and the ova and sperms are shed into the atrium and escape to the exterior by the atriopore, or it may be in some cases by the mouth.
Embryology and Life-History.
Development takes place in the sea-water where the egg is fertilised—apparently always about sunset, the embryonic stages being passed through during the night, and the larva hatched in the early morning.
Fig. 82.—Stages in the segmentation of Amphioxus. A represents the eight-celled stage; B, the sixteen-celled; D, vertical section of C; F, vertical section of the blastosphere or blastula stage (E). (From Korschelt and Heider, after Hatschek.)
The egg is small (0.105 mm. in diameter when shed) and contains very little food-yolk. Segmentation is complete (Fig. 82, A), is nearly regular, and results in the formation of a hollow blastosphere (Fig. 82, E, F), the wall of which is one cell thick. The lower cells (Fig. 82, B, C, D) are slightly larger than the upper. Invagination of the lower cells then takes place (Fig. 83, A), resulting in the suppression of the blastocoele or segmentation cavity and the formation of an archenteron, at first shallow and opening widely to the exterior (Fig. 83, B), and then deeper and with the opening narrowed to a small posterior blastopore (Fig. 83, C). This "gastrula" stage differs from the blastosphere in having a mouth or blastopore, and in being two cell-layers thick—epiblast (ectoderm) on the outside and hypoblast (endoderm) within. It soon shows the future aspects of the body by its elongation and shape (Fig. 83, C), as the dorsal surface becomes flat and the ventral convex, while the blastopore is at the posterior end of the dorsal surface. The blastopore soon closes, and the mouth and anus are formed independently later.
