IN THE LARV^ OF ANISOPTERID DRAGOKFLIES. 14)^ 



carrying in its meshes the efferent trachea and its main branches for that 

 particiihir portion ol: the gill-fold under consideration. Generally speaking, 

 the hi/pohrancMal tissue is flanked on one or both sides by the basal pad, from 

 which it is easily distinguished at once by its different structure, nuclei, and 

 staining qualities (see p. 176). 



71ie Circular Muscle Tunic. — The outstanding feature of the circular 

 muscle-layer of the gill-basket is its weakness. It is everywhere composed 

 of only one row of fibres, which run closely parallel around the basket so 

 as to form a complete investment, except for numerous spaces left for the 

 tracheae to pass through (text-fig. 1, mcr). The weakness of this tunic pre- 

 cludes the supposition that it is of use in the contraction or expansion of the 

 gill-basket. These movements appear to be caused by the movements of 

 the segmental muscles of the abdomen, and in particular by the six radial 

 muscles already mentioned. 



The Longitudinal Muscle- Bands. — In the young larva each longitudinal 

 band is seen to be composed only of five or six strands of muscle-fibre placed 

 fairly close together so as to form a definite band (text-fig. 1, ndr). In more 

 mature larvae a larger number of fibres is present, and the longitudinal bands 

 become very definite structures. They do not seem, however, to be of much 

 strength considering the size of the gill-basket. They aid, by their con- 

 tractions, the shortening of that organ which takes place regularly during the 

 act of rectal breathing ; but their function can only be regarded as auxiliary 

 to that of the far more powerful segmental muscles of the abdomen. 



STUDY OF GILL-TYPES. 

 A. Simplex System. 



Definition. — The Simplex System of gills consists of six holobranchs in 

 the positions 2, 4, 6, 8, 10, 12 of the clock-face. Each holo branch is a con- 

 tinuous longitudinal eversion of the rectal wall, supported to right and left 

 alternately by numerous small cross-folds, these latter lying in a slanting 

 direction nearly transverse to the main fold, and not projecting far into the 

 rectal cavity. 



TJie Tracheal Supply. — We have already mentioned that each primary 

 efferent trachea divides into two secondary efferent trachece before reaching 

 the branchial basket. Thus there are tivelve sets of secondary efferetit tracheae 

 entering the basket from outside. There are also tioelve sets of cross-folds, 

 viz., one set to right and one set to left of each main longitudinal fold. 

 Now these cross-folds lie below the courses of the secondary tracheae — in 

 fact, each cross-fold is developed in relation to a single secondary trachea, 

 which sends out branches and capillaries into it. Each secondary trachea, 

 however, continues inwards in a slanting direction until it reaches a mam 

 longitudinal fold, into which it gives off" branches and, finally, capillaries. 



