Fig. 244. Section through the procephalic lobes of an embryo of Agelena labyrinthica.
st. stomadæum; gr. section through semicircular groove in procephalic lobe; ce.s. cephalic section of body cavity.
Fig. 245. Two transverse sections through the embryo of Hydrophilus. (After Kowalevsky.)
A. Transverse section through an embryo in the region of one of the stigmata.
B. Transverse section through an older embryo.
vn. ventral nerve-cord; am. amnion and serous membrane; me. mesoblast; me.s. somatic mesoblast; hy. hypoblast (?); yk. yolk-cells (true hypoblast); st. stigma of trachea.
Full details as to the development of the nervous system in the Crustacea are still wanting; a fairly complete account of what is known on the subject is given in Vol. II. pp. 521-2. It appears that the ventral cord may either arise as an unpaired thickening of the epiblast (Isopoda), marked however by a shallow median furrow, or from two cords which eventually coalesce[154]. It is not certain how far the supraœsophageal ganglia are usually in the first instance continuous with the ventral cord. In Astacus, the early stages of which have been elaborately investigated by Reichenbach (No. [331]), they are stated to be so; the supraœsophageal ganglia are moreover described by this author as having a somewhat complicated origin. Five elements enter into their composition. There is first formed a pair of pits on the procephalic lobes, which become very deep during the Nauplius stage, and are continuous with a pair of epiblastic ridges which pass round the mouth, and join the ventral cords just described. The walls of the pits are believed to form a part of the embryonic ganglia which gives rise to the retina as well as to the optic ganglia. The ridges form the remainder of the ganglia and the œsophageal commissures; while the fifth element is supplied by a median invagination in front of the mouth, which appears at a much later date than the other parts.
In the Isopoda supraœsophageal ganglia are stated to arise as thickenings of the procephalic lobes, which become eventually detached from the epidermis.
The ventral cord is at first unsegmented, but soon becomes partially divided by a series of constrictions into a number of ganglia, corresponding with the segments. The development of the commissural and ganglionic portions takes place much as in the Chætopoda.
The Gephyrea approach closely the types so far dealt with, but the ventral cord in the Inermia is formed as an unpaired thickening of the epiblast. In Echiurus, as has been shewn by Hatschek in an interesting paper on the larva of this species, published since the appearance of the first volume, there is a pair of ventral cords[155]. In correspondence with a general segmentation of the body, which is subsequently lost, these cords become segmented. The two cords unite in the median line, and Hatschek, in accordance with his general view on this subject, states that their junction is effected by means of a median cord of invaginated epiblast. The segmentation of the cords subsequently becomes lost. The supraœsophageal ganglia arise as an unpaired median thickening of the procephalic lobe. No traces of segmentation in the ventral cord have been observed by Spengel in Bonellia, and the supraœsophageal ganglion is formed in this genus as an unpaired band.
In all the groups above considered the nervous system clearly presents the same type of development with various modifications.
It is formed of two parts, viz. (1) the supraœsophageal ganglia, and (2) the ventral cord.