90 ARACHNIDA. 



eardrum, while the anterior and posterior aortae, as well as the 

 lateral arteries, originate as prolongations of the heart or as out- 

 growths from it (Schimkewitsch). 



While the cavity of the heart appears to be a part of the primary body-cavity, 

 the pericardial space, according to Schimkewitsch, corresponds to a part of 

 the secondary body-cavity. The pericardium in the Arachnida forms a tube, 

 and is not comparable with the synonymous structure in the Insecta. But 

 before we can make any definite statement as to the nature of the pericardium 

 we must have a more exact account of its origin. 



The Body- cavity. In the Arachnida, as in other Arthropoda, the 

 hlood-vascular system is not separated from the hody-cavity, but 

 the latter is directly connected with the circulation of the blood. The 

 method of development of the body-cavity in the Arachnida is, 

 however, strikingly different from that in the Crustacea, Myriopoda, 

 and Insecta. While, in these latter, the primitive segments are not 

 large and soon undergo degeneration, in the Arachnida they are 

 almost as largely developed as in the Annelida (Figs. 45 and 46). 

 The primitive segments are also highly developed in Peripatus to 

 begin with (Fig. 100), but this form resembles the Insecta in that 

 the segments very soon cease growing, and after a rich growth of 

 cells undergo early disintegration. The adult body-cavity forms (as 

 a pseudocode) outside the primitive segments. In the Arachnida 

 it forms somewhat differently ; it is, however, difficult, from the 

 statements before us, to arrive at a satisfactory judgment, since 

 little stress has until now been laid upon this point. It is certain, 

 however, that the primitive segments are of considerable size even at 

 a somewhat advanced stage of development (Figs. 46 and 47). 

 Between the somatic and splanchnic layers of each primitive 

 segment there is a rather large cavity, and we must assume that 

 when the union of the segmental cavities takes place this passes 

 direct into the adult body-cavity. It is true that here, also, the 

 body-cavity would not retain the coelomic epithelium up to the last, 

 but the wall of the primitive segments would also break up (Figs. 

 47 A and B, 41, p. 80, 42, p. 82), yielding the muscular and con- 

 nective tissue elements, so that at last, in the Arachnida, a condition 

 would be reached similar to that attained at a much earlier stage in 

 the development of the Crustacea, Myriopoda, and Insecta. 



The segmentation of the mesoderm begins to disappear when the 

 primitive segments have grown to a considerable size and the 

 embryo itself is near the stage illustrated in Fig. 27. The segmental 

 cavities unite in the cephalo-thorax and the dividing walls (dissepi- 



