THE ARTHROPODA 95 



and smooth in those annuli which are covered by the cephalic 

 shield, but are thicker and furnished with backwardly directed 

 chitinous spines in the posterior region which projects beyond 

 the cephalic shield. In the last four annuli the chitinous 

 hoops are complete ventrally, and do not bear any limbs or 

 appendages. The furcal styles borne by the last segments may 

 or may not represent appendages belonging to the same series 

 as those about to be described. 



A glance at the ventral aspect of Apus shows that the 

 animal is provided with a large number of limbs or appendages, 

 which are borne on the under surfaces of all the annuli but the 

 last four, and some appear to belong to the head region. 

 Closer examination shows that in front of the mouth there are 

 two pairs of minute pre-oral appendages, generally described 

 as the first and second antennae, and behind the mouth there 

 are sixty-six pairs of appendages. Of these the first three 

 pairs lie close together round the mouth, and, being modified 

 for masticatory purposes, they are usually known as foot-jaws. 

 The following sixty-three pairs are functional limbs, but they 

 are not all alike, and may be divided into thoracic or pregen- 

 ital, genital, and postgenital series. There are ten pairs of 

 thoracic limbs attached to a corresponding number of annula- 

 tions of the body. The single pair of genital limbs is attached 

 to the annulus on which the generative organs open. The 

 fifty-two pairs of postgenital limbs are borne on seventeen 

 annuli only, the first four of these carrying five pairs of limbs, 

 the next four ten pairs, the next four thirteen pairs, and the 

 last five carry twenty-four pairs of limbs. The more posterior 

 appendages are soft, foliaceous, many-lobed plates which over- 

 lie one another like the leaves of a half-opened book, but the 

 anterior members of the thoracic series are modified and 

 the foot-jaws and antennae do not appear to bear any resem- 

 blance to the limbs behind them. We shall do well to take the 

 first postgenital limb, the seventeenth of the whole series of 

 appendages, as a type, and we will choose that of the right side. 



It consists of a median axis or corm, flattened antero- 

 posteriorly and attached by one end to the body near the mid- 

 ventral line in such a way that one of its narrower edges looks 

 outwards and upwards, the other inwards and downwards. The 

 axis is invested by a chitinous cuticle, which is thinner at the 

 junction with the body-wall, so that an imperfect joint is formed 



