44 



idea of the form of any object, corresponding, as they do, 

 to the three dimensions in space." It seems more likely, 

 hoAvever, that it functions as an olfactory organ, if 

 it be safe to reason from analogy with what is known 

 concerning the antennal sense organs of the Pterygota. 



The sensory hairs are found principally at the apices 

 of the antennae, and on the upper and lower lips, but 

 possibly some of the hairs scattered over the general 

 surface of the body may be of a sensor}^ nature al^o. 



10. The Circulatory System. 



The circulatory system of Anurida consists of the 

 heart and the general body-cavity. 



The heart is a narrow tube situated just beneath the 

 integument in the median dorsal line of the body above 

 the digestive canal (Plate lY., fig. 35). It is divided by 

 means of segmeutally arranged constrictions iuto a series 

 of six consecutive chambers, which are in free communi- 

 cation with one another. Anteriorly, the heart extends 

 as far as the anterior border of the mesothoracic segment, 

 and from that point it is prolonged forwards as the 

 aorta [a.). Posteriorly, it increases in calibre, and 

 terminates rather abruptly in an enlarged and somewhat 

 bulb-like chamber in the fourth abdominal segment 

 [a. bib.). Situated on either side of the heart at each of 

 the constrictions is an oval oriface or ostium (ost. in 

 fig. 41), and in association with the heart at these same 

 points is a pair of alary muscles (///. m.). 



The walls of the heart consist of an external invest- 

 ment of connective tissue {ic. in fig. 40), a middle coat of 

 obliquely disjDOsed muscle fibres {c.m.b. in fig. -59 and ;/(.»'. 

 in fig. 40), and an inner lining or endocardium. The 

 muscle layer consists of bands of non-striped muscle 

 fibres, and is the only part of the body where such are 



