1. The legs are not of equal length, the posterior legs being the longest, as in Chilopods.
2. The mouth parts differ from those of Chilognaths almost as much as from those of Chilopods.
3. The form of the antennae.
Only a few Pauropoda have been discovered as yet.
Embryology.
The preceding account of the anatomy of Myriapods would be incomplete without some reference to the wonderful manner in which the different organs of the body are built up; the whole of the complex organism proceeding by a gradual and regulated process of development from a simple cell called the ovum derived from the female body, and united with a cell from the male body (called the spermatozoon). I hope to be able to give my readers some idea of the interest which the pursuit of the difficult study of embryology adds to anatomy, by offering us a key to the interpretation of the relations between our knowledge of the forms at present living on the earth and those which, we learn from Palaeontology, have inhabited our planet in past ages.
Fig. 36.—Young ovum of Julus terrestris: nucl, nucleolus; nu, nucleus; R, first appearance of yolk; F, follicle cells.
Like all living creatures with which we are acquainted, the starting-point of Myriapod life is the ovum, as it is called. This ovum is a cell resembling the cells of which the body of all living animals are built up, and which may be compared to the bricks of which a building is composed. This cell or ovum is a small sphere of living transparent substance called protoplasm, and it is nucleated—that is, it contains a small spot of denser protoplasm called the nucleus, and within that a still smaller spot of still more dense protoplasm called the nucleolus. In the process of impregnation the ovum unites with the male cell, and the cell so formed is called the impregnated ovum. This ovum has the property of dividing into two cells, each resembling the parent cell from which it is derived; each of these cells has, like the parent cell, the same property of dividing into two more, and so on. Thus from this continual process of division or reproduction of every living cell, the materials are provided for the building up of the body.
The regularity of the process of the division of the ovum, or, as it is called, segmentation of the ovum, is interfered with by the presence of food yolk. The cells formed by the process of cell division just described need nourishment, and this nourishment is supplied to them by the food yolk formed in the body of the ovum before the process of segmentation begins. It is easy to understand that this yolk, which is not alive like the cells, cannot divide like them, and therefore the segmentation of the ovum in Myriapods is irregular, as it is called.