226 ZOOLOGY FOR MEDICAL STUDENTS chap. 



consist almost entirely of a massive gnathobase, the rest of the limb 

 being represented by a small 3-jointed endopodite (Fig. 103, M, en) 

 which acts as a palp or sensory organ. In the appendages anterior to 

 the second maxilliped the gills have disappeared, but the plate-like 

 epipodite is still present on the first maxilliped and the second maxilla. 

 In the latter the epipodite is continuous with the exopodite which is also 

 plate-like in character, the two together constituting what is known 

 as the scaphognathite (Fig. 103, Mx.2, ep. + ex). 



The scaphognathite performs an important function in respiration. 

 It lies in the front portion of a chamber into which project the gills, 

 covered in and protected by a flap-like downgrowth of the dorsal body- 

 wall — the branchiostegite or gill-cover, During life the scaphognathite 

 performs rhythmic movements of such a kind as to draw a current 

 forwards through the chamber so that the water bathing the respiratory 

 surface of the gill undergoes constant renewal. 



In front of the mouth lie two pairs of antennae. These are prolonged 

 into one (second antenna) or two (first antenna or antennule) long, taper- 

 ing, many-jointed filaments which are crowded with sensory cells and 

 clearly function as organs of sense. They naturally recall to memory the 

 sensory tentacles borne by the prestomium of the annelid, but the study 

 of their development shows that they are quite different in their nature. 

 The antennae of arthropods are true appendages which have become 

 shifted forwards, so as to lie in front of the mouth, and taken on a purely 

 sensory function. 



Appendages of Insecta 



As has already been remarked, the insects are normally without 

 appendages on the abdomen. In the thoracic region there are present 

 three pairs of highly developed walking legs each terminating in a foot 

 or tarsus consisting of a number (5 or less) of small segments, the 

 terminal one carrying a pair of claws. Between the claws there may 

 be present a cushion-like pad, the surface of which produces a sticky 

 secretion enabling the insect to creep on smooth vertical or inverted 

 surfaces. 



The appendages in the neighbourhood of the mouth are modified in 

 connexion with the act of feeding and — in correlation with the great 

 differences between different types of insects in the nature of their food 

 and the manner of feeding — great differences exist between the mouth 

 appendages of different insects. Thus in the Bees the mouth-appendages 

 are adapted for collecting pollen and nectar from flowers ; in the Mosquitos 



