EXTERNAL STRUCTURE OF THE ORAL LOBES 15 



before. Distally, the hypopharyngeal tube becomes free from 

 the labium, as shown in fig. 29, and ends in a point where the 

 lingual salivary duct opens. 



Down each side of the labium-hypopharyngeal sclerite a 

 rod-like thickening runs. Distally, these thickened margins 

 (paraphyses of Lowne) articulate with the discal sclerites. The 

 discal sclerites (fig. 3, ds.) are united at the posterior end ti» 

 form, when the oral lobes are expanded, a U-shaped structure, 

 with the limbs constricted in the middle where the ends of the 

 thickened margins of the labium-hypopharynx articulate. They 

 are sunk in deeply between the two oral lobes at the base of the 

 oral pit with the free ends of the U anterior, these being spatulate 

 and curved anteriorly. 



The Oral Lobes. Normally the two oral lobes or labella are 

 connected by a delicate attachment along the inner anterior edges 

 to form an oral sucking organ, but under pressure this delicate con- 

 nection is severed and the oral disc presents a heart-shaped instead 

 of the normal appearance. On the upper or outer aboral surfaces 

 the oral lobes bear sensory setae, the larger marginal setae being 

 different in structure from the rest, as will be described later in 

 the account of the internal structure of the oral lobes (p. 61). 

 On the lower and, when the proboscis is withdrawn, the inner 

 oral surface a large number of channels, called the pseudo- 

 tracheae (fig. 3, ps.), from their fancied resemblance to the 

 annular tracheae, run from the edges of the oral lobes to the 

 internal margins. These channels are almost circular or oval in 

 section, being incomplete on one side and thereby communicating 

 with the surface of the oral lobe. The channels are kept open 

 by means of small incomplete chitinous rings which give the 

 pseudo-tracheae their annular ajDpearance. Each of these incom- 

 plete chitiiious rings is bifurcated at one end but single at the 

 other end (fig. 4). The rings are so arranged that the bifid 

 ends alternate with the single ends. The pseudo-tracheal channel 

 communicates with the external surface of the oral lobes through 

 the opening through the bifid extremities of each ring, as is 

 shown in the accompanying figure (fig. 5). From the outer 

 edge of the oral lobe the pseudo-tracheae gradually increase in size 

 as they approach the inner margin of the lobe. The number 



