BY R. J. TILLYARD. 427 



(2). The basal pad. — Not only is this developed at the base of 

 the gill, but it is either absent or very feebly developed in gills 

 of the Implicate Duplex Type, as well as in the gills of the 

 majority of young larvae. Hence we may not only class it as an 

 accessory, but we may gather that its chief function is that of a 

 support for the gill. For, in the implicate duplex type, the gills 

 overlap one another in the form of a series of concave tiles with 

 rounded ends, these ends being connected together so as to form 

 a continuous wavy ridge. Thus the two rows of gills in one 

 double-row support one another, and no basal pad is needed. 

 The fact that the basal pad is absent or vestigial in this type of 

 gill points, therefore, to its functioning in general as a support 

 for the gill. 



(3). The hypohranchial tissue. — This also is absent or feebly 

 developed in young larvae, and also in gills of the Implicate 

 Duplex Type, as well as being developed, in other forms, only at 

 the extreme base of the gill. Hence we may class it as an acces- 

 sory. It is possible, however, that this tissue may play some 

 part in the physiology of respiration, apart from that of oxygen- 

 extraction. It may, for instance, help in the removal of nitrogen 

 or carbonic acid gas. 



(4j. The blood-plasma. — We class this as an accessory, partly 

 because there is no definite blood-circulation in the gill itself, 

 and no clearly defined blood-spaces except at its base, and partly 

 because the blood of insects (except in a few very special cases) 

 appears to be entirely lacking in oxygen-carrying elements. We 

 must, however, accord to the blood an important part in the 

 removal of carbonic acid gas from the body, and perhaps also 

 portion of the nitrogen. 



Thus, out of eight elements of the gill-structure, four are seen 

 to be accessories. We must next remove from the problem a 

 fifth, viz., the larger tracheae, since these clearly function only as 

 receivers of oxygen from the capillaries, i.e., eiferents to the main 

 tracheal system. Thus the problem is now reduced to the con- 

 sideration of three elements, viz., the cuticle, the ejnthelial 

 syncytium (minus the pigment), and the tracheal capilla7nes. 

 How do these three combine for the extraction of oxygen from 

 the water in the rectal cavity ? 



