160 THE CIRCULATORY SYSTEM [CH. 



organs lie just at the points where the blood-current is most 

 intense. Their functions appear to be (1) the manufacture of 

 fresh blood corpuscles, (2) the extraction of certain impurities 

 from the blood stream. A pair of tymphoid organs also lie between 

 the heart and the pericardium, opposite the ostia, and appear to 

 pulsate with the heart. 



The Conductive Heart consists of six chambers (iii to viii) lying 

 in segments 7 to 2. All six are very closely similar. Each is 

 cylindrical, and opens into the one before it by means of a pair of 

 valves (v). There are no alary muscles, and no ostia. Hence the 

 function of the Conductive Heart is purely ventricular ; and even 

 in this its action is not so powerful as that of the Hind Heart. 



Replacing the absent ostia, on the wall of each chamber of 

 the Conductive Heart, is a pair of peculiar oval organs, called by 

 their discoverer Zawarsin "ostia-organs." As they are almost 

 certainly reduced and closed-up ostia, we shall use the term osteoles 

 for them. These osteoles (ol) lie close together on the dorsal wall 

 of each chamber, a little in front of the middle. There is also 

 a single pair on the aorta. Zawarsin believes that they have been 

 formed by the fusion of an original phagocyte organ with an 

 ostium proper. Consequently their chief function must be the 

 manufacture of fresh blood corpuscles. 



The Conductive Heart narrows slightly from behind forwards. 

 In the second segment it ends abruptly, and passes by a pair of 

 valves into the very narrow aorta (d.aort.). 



Surrounding the whole heart, but only distinctly separated 

 from it around the Hind Heart, is the Pericardium (pc), a space 

 bounded by a delicate cellular wall supported by fibres of the 

 alary muscles. On reaching the region of the heart, the muscle 

 proper becomes differentiated into the so-called "elastic tissue." 

 This, by interweaving of its fibres with the pericardial epithelium, 

 forms a kind of net-bag in which the hind-heart lies free, except 

 ventrally, where the fibres pass on into*the wall of the heart itself. 

 Specially thickened portions of this elastic tissue form the anterior 

 and posterior ligaments. 



In the Fore Heart, the pericardial wall fuses completely with 

 the wall of the heart, so that no distinct pericardial cavity is 

 recognizable. 



