VI AKTENNATA—THE CIRGULATORY SYSTEM 475 



Beneath each olfactory process there is a pore in the cuticle, through 

 which the fibrillas of a hypodermal ganglion pass (Fig. 335). Similar 

 terminal apparati, which have been observed at the base and at the 

 point of the tongue, and on the lower side of the maxillae of Hymeno- 

 ptera, on the inner sui-face of the labellum of the proboscis in the fly, 

 and in the gnathochilarium of the Diplopochi, may perhaps be con- 

 sidered as gustatory organs. 



Speeifie organs of touch are found in the long, simple, or feathered 

 setse (Fig. 330, p. 472) which occur all over the body surface, and 

 especially on the feelers and maxillar palps of the Antennata, on the 

 labellum of the proboscis of the fly, etc. An axial thread enters the 

 sensitive hair from a hypodermal ganglion cell, and runs through it. 



VII. The Circulatory System. 



This is very simple in the Antennata. The colourless or light- 

 yellow or light-green blood, containing amoeboid blood corpuscles, flows 

 in definite dkections in a lacunar system (body cavity). The circula- 

 tion of the blood is maintained by the contractions of a dorsal vessel 

 (dorsal heart), which in the extremely reduced condition of the 

 arterial vascular system is almost the only part which has walls of its 

 OAvn. The dorsal vessel is a delicate tube running longitudinally above 

 the intestine, and covered externally and lined internally by membranes 

 probably elastic. Between the two membranes there runs a system of 

 delicate muscle fibres, which generally have a circular course and 

 sometimes cross each other. The dorsal vessel falls into successive 

 segmental chambers, separated by valvular arrangements, which prevent 

 the streaming back of the blood from the anterior into the posterior 

 chambers during the progressive contraction of the vessel from behind 

 forward. The dorsal vessel is provided with paired lateral ostia, 

 which, as it appears, are mostly placed intersegmentally, and maintain 

 an open communication between the surrounding parts of the body 

 cavity and the interior of the heart. The heart may be connected in 

 various ways by muscular fibres mth neighbouring parts of the body, 

 i.e. with the intestine, with the dorsal integument, etc. Paired so- 

 called alary muscles are particularly constant in their occurrence ; 

 they are almost triangular, and become attached by their narrowed 

 ends to the latero-dorsal body wall, while their broad ends are fastened 

 to the chambers of the heart. These alary muscles together ' form an 

 incomplete horizontal partition wall above the intestine, marking ofi' 

 a dorsal sinus in which the heart lies. This sinus may be called the 

 pericardial sinus. This partition wall is arched upwards, and if 

 the alary muscles which form it contract, it straightens out and 

 becomes flat ; the pericardium thus becomes more spacious, > and the 

 blood streams into it from the rest of the body cavity. It was 

 formerly thought that the alary muscles served to expand the heart. 



