SKELETON AND BODY WALL 445 



is bounded ventrally by the thick lateral edge of the hypostome, and posteriorly by the 

 cuticular thickening which runs up between the maxillule and maxilla to the upper 

 end of the posterior hypostomal strut. 



The mandibles and maxillules are thus firmly connected to the adductor muscle by 

 the endoskeletal apodemes and labrum. The remaining limbs, with the possible excep- 

 tion of the first trunk limbs, are connected with the adductor system through a series 

 of articulating sclerites which, until a more detailed and comparative account of the 

 skeleton can be given, I am denoting by letters. 



A thin sclerite labelled a (Fig. 2) runs antero-dorsally from the upper margin of the 

 attachment of the adductor muscle. It articulates with a shorter sclerite b, which, in 

 its turn, articulates with sclerite c which is shaped like an inverted Y. This runs down 

 towards the labrum. The anterior branch of the Y reaches a point just above and at 

 the side of the frontal knob. It articulates directly with a thin sclerite running along 

 the median face of the first joint of the antennule. The posterior branch similarly sup- 

 ports the inner attachment of the antenna to the body. It stretches down to the com- 

 mencement of the antenno-labral apodeme. At the junction of the a and b sclerites a 

 fourth sclerite d slopes downwards and supports the outer attachment of the antenna 

 to the body. 



The lateral eyes are attached to the body by short eye stalks close against the front 

 of the upper ends of the c sclerites. Their movement is effected through muscles which 

 attach to these anterior sclerites. 



From the posterior side of the attachment of the adductor muscle to the body, a 

 thin sclerite e runs downwards and supports the attachment of the second trunk limb. 

 Dorsally it is connected with the base of the a sclerite by a powerful apodeme which 

 runs anteriorly just below the dorsal constituent of the adductor muscle. 



Postero-ventrally to the adductor attachment a thin sclerite /, pointed at both ends 

 with a posterior knob projecting about the middle of its length, runs down to the end 

 of the thickening which has already been described as the continuation up the side of 

 the body of the posterior hypostomal strut. On the posterior knob a small rod-like 

 sclerite articulates, and supports the upper part of the vibratory plate of the maxilla. 



The first trunk limbs are attached to the body, very close together in the middle 

 line. The body wall, both in front of them and behind, is folded inwards so as to form 

 a transverse ridge (Figs. 2, 3), but there does not appear to be any special sclerite sup- 

 porting the attachment. 



ENDOSTERNITE 



The true endoskeletal elements exhibit a characteristic staining reaction in Mallory. 

 On the whole they stain a fairly deep Cambridge blue, but always a part stains 

 bright orange (Plate VII), which is the colour taken up by the exoskeletal sclerites. 

 The endosternite, on the other hand, stains a bright royal blue, and can be clearly 

 distinguished from the endoskeletal elements. It is of a different composition from the 

 latter, as it disappears completely after treatment with potash. 



