TRACHEATA 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 
