TR AC HE AT A 303 



recalls the modified segmental organs which open into the oeso- 

 phagus in some of the Oligochaets. The pharynx is very 

 muscular, and has a triradiate lumen ; it is strikingly unlike the 

 same region of the digestive tube in any other Arthropod, but 

 shows considerable resemblance to the pharynx of many of the 

 Chaetopods. It communicates by a short oesophagus with a 

 large stomach, which extends from the level of the second pair 

 of legs to nearly the posterior end of the body. The stomach 

 lies quite freely unsupported by any mesentery ; it ends in a 

 short muscular rectum, which passes to the anus. The 

 pharynx, oesophagus, and rectum are all lined with chitin con- 

 tinuous with that covering the body. 



The vascular system consists of a tubular heart, which lies 

 between the two dorsal muscle bundles along the whole length 

 of the body. Paired ostia situated two in each segment, on 

 the dorsal surface of the heart, put the cavity of this organ in 

 communication with the surrounding pericardial space. The 

 pericardial membrane which limits this space is attached on 

 each side to the body-wall ; it forms a kind of meshwork with 

 numerous holes, by means of which the central division of the 

 body -cavity communicates with the pericardium. 



The space spoken of above as the body-cavity in which 

 the chief organs of the body lie, is not a true coelom. That 

 is, it does not communicate with the exterior by means of 

 nephridia, the lining of its walls does not give rise to the 

 generative cells, and it does not arise as a definite split in the 

 primitive mesoblastic somites. A true coelom is found in Peri- 

 patus, but it has a different fate, and the space mentioned 

 above as the body-cavity has nothing to do with it. The 

 latter space originates either as a split between the endoderm 

 and ectoderm, or as a hollowing out of spaces which appear 

 in the thickened somatic walls of the somites. Speaking 

 broadly, the central compartment and the heart arise in the 

 former manner, the lateral spaces, with the cavities in the 

 appendages and the pericardium, arise in the latter. These 

 various cavities all communicate with one another. Thus the 

 heart and the various portions of the body-cavities form a series 

 of spaces which have been termed haemocoelic. They have 

 nothing to do with the coelom, but are "a series of enormously 



