THE REGION OF THE SPINAL CORD 431 
That the vertebrate body-cavity was originally a nephroccele is 
generally accepted, and its excretory function is shown by the fact 
that it communicates with the exterior in all the lower vertebrates, 
either through abdominal pores or by way of nephridial funnels. 
Bles has shown how largely these two methods of communicating 
with the exterior mutually exclude each other. In the higher verte- 
brates both channels become closed, except in the case of the 
Fallopian tubes, and thus, so to speak, the body-cavity becomes a 
ductless gland, still, however, with an excretory function, but now, 
as in all other cases, forming a part of the lymphatic rather than of 
the true excretory system. 
SUMMARY. 
The consideration of the formation of the vertebrate cranial region, as set 
forth in previous chapters, indicates that the ancestor of the vertebrates was 
not an arachnid purely or a crustacean purely, but possessed partly crustacean 
and partly arachnid characters. In order to express this conclusion, I have 
used the term Protostraca, invented by Korschelt and Heider, to indicate ‘a 
primitive arthropod group, from which both arachnids and crustaceans may be 
supposed to have arisen, and have therefore stated that the vertebrate did not 
arise directly from the annelids, but from the Protostraca. Such an origin 
signifies that the origin of the excretory organs of the vertebrate must not 
be looked for in the segmental organs of the annelid, but rather in such 
modified annelid organs as would naturally exist in a primitive arthropod 
group. The nature of such organs may be inferred, owing to the fortunate 
‘circumstance that so primitive an arthropod as Peripatus still exists, and we 
may conclude that the protostracan ancestor possessed in every segment a pair 
of appendages and a pair of ccelomic cavities, which extended into the base of 
these appendages. The ventral portion of each of these ceelomic cavities 
separated off from the dorsal and formed a nephroceele, giving origin to a 
segmental excretory organ, which, seeing that its end-vesicle was in the base 
of the appendage, and seeing also the nature of the known arachnid and 
crustacean excretory organs, may fitly be termed a coxal gland. This, then, 
is the working hypothesis to explain the difficulties connected with the origin 
of the pronephros and mesonephros—that the original segmental organs were 
coxal glands, and therefore indicated the presence of appendages. This 
hypothesis leads to the following conclusions :— 
1. The coxal glands belonging to the post-branchial appendages of the 
invertebrate ancestor are represented by the pronephric tubules, and existed 
over the whole metasomatic region. 
2. Such glands discharged into a common duct—the pronephric duct— 
which opened into the cloacal region, either in the protostracan stage, when 
the metasomatic appendages were still in existence, just as the coxal glands 
of the prosomatic region in Limulus discharge into a common duct, or else the 
pronephric duct was formed when the appendages were obliterated. 
