IX ] THE RESPIRATORY SYSTEM 199 



The above description applies to the sacci of Diphlebia and 

 Neosticta, which I have examined. Bis [137] has made a very 

 careful study of the saccus of Pseudophaea, which differs, appa- 

 rently, from the above description in possessing within its alveoli 

 peculiar branching tufts of fibrils, shewing beautiful mossMike 

 designs. 



The lamellar gill (fig. 86 c, D) is of simpler structure. The 

 cuticle (CM) resembles that of the saccus in being unpigmented, 

 but it does not usually carry hairs. The cuticle is thickened along 

 the mid-rib (m), which is developed only on the outer side of the 

 lateral gill, but on both sides of the median gill. Beneath the 

 cuticle is a layer of hypoderm cells (%). These, in the most 

 flattened portions of the lamella, resemble the hypoderm cells of 

 the saccus. Where the gill is thickened, the hypoderm cells become 

 elongated, and tend to assume a pyramidal form, with clear spaces 

 interposed along their inner ends. The pigment is nearly all 

 deposited in the layer of protoplasm lying closest to the cuticle, 

 where also the nuclei are situated. The alveoli are much reduced 

 in number, but can still be seen to be present, especially in the 

 thickened part of the gills, under the mid-rib. 



The inner ends of the hypoderm cells rest on a definite basement 

 membrane. The basement membranes of the two opposite walls 

 of the lamella are contiguous in many places. In others they are 

 separated by blood-channels or extensions of the haemocoele (h), 

 or to allow of the passage of tracheae (tr, tr'). Either one or two 

 main longitudinal tracheae (tr) run close under the mid-rib ; in the 

 median gill there are always two. The various types of branching 

 of the tracheae from this main stem have been already described 

 and figured. 



It is instructive to compare the structure of a cross-section of 

 a lamellar gill with that of a larval wing-sheath. The two organs 

 are, histologically, somewhat similar, yet they serve two very 

 different purposes. 



Our knowledge of these caudal gills is insufficient to allow of any 

 speculations as to their physiology. Some authors, indeed, deny 

 their respiratory function, and will only allow that they act as a 

 kind of tail-fan, after the manner of the uropods and telson in 

 many Crustacea. We know that many larvae use their caudal 



