THE BODY-CAVITY AND THE BLOOD-VASCULAR SYSTEM. 203 



The lower of these spaces (m.lh) at a later stage grows above the 

 intestine, and unites with its fellow and with the space which has 

 already appeared beneath the intestine (Ih) to form the greater part 

 of the permanent median cavity, the so-called central compartment 

 of the body-cavity, while the upper one represents (pe) the rudiment 

 of the pericardial cavity. 



The pericardial spaces on each side extend towards the median 

 line, the remains of the primitive segments being thus displaced 

 downwards. The cavity (lit) that arose early above the intestine 

 thus appears confined, together with the dorso-median portions of 

 the primitive segments (s7i), between the two pericardial spaces (pe) ; 

 the latter grow above the pseudocoele (Ih), and also between the 

 latter and the dorsal portions (sh) of the primitive segments, and 

 unite with one another in the middle line. Thus the common 

 pericardial cavity is formed, surrounding the dorsal pseudocoele (Ih) ; 

 the latter now assumes a tubular form and becomes the definitive 

 heart (Fig. 102, h). According to Sedgwick, the primitive segments 

 take no part in the formation of the heart. The ostia of the heart, 

 the formation of which has not been closely observed, do not arise 

 until later, when the embryo is ready for birth. 



Detached mesoderm-cells, which become applied to the outer wall of the heart, 

 give rise to the cell-mass within the pericardial cavity, which has been compared 

 to the fat-body of the Insecta. It involuntarily reminds us of the cell-growth 

 on the dorsal vessel of the Annelida, which is probably homologous with the 

 pericardial gland of the Mollusca ; but we are prevented from homologising 

 the two structures because the pericardial gland, as an outgrowth of the 

 peritoneal epithelium, lies within the secondary body-cavity, while the cell- 

 mass in Peripatus lies outside the latter. The pericardial space in Peripatus, 

 like that of other Arthropods, does not correspond to the pericardium of the 

 Mollusca or the coelom of the Annelida. Only its ventral wall (the pericardial 

 septum, Fig. 101 B, and 102, ps) is perhaps in part formed by the somatic wall 

 of the primitive segments, as is also the case in the Insecta. In Peripatus, as 

 in the Arthropoda, the dorsal vessel is in direct communication, in the adult, 

 with the body-cavity, and this fact is explained by the similarity in the 

 development of this system of organs in the two divisions. 



In the two anterior (cephalic and maxillary) segments, the trans- 

 formation of the primitive somites undergoes certain modifications 

 determined by the special form of these parts. 



In the maxillary segment, the inner or dorsal part of the primitive 

 somite is not extensive, and fuses with the corresponding part of the 

 succeeding segment which projects into this segment. The different 

 spaces of the permanent body-cavity are here less distinctly developed. 

 The lateral parts of the primitive segments which occupy the rudi- 



