ries of archenteric pouches, like
those of Actinozoa and Medusae; (3) that excretory organs (nephridia,
segmental organs) are derived from parts of these pouches which in the
ancestral form, as in many polyps, were connected by a circular or
longitudinal canal, and opened to the exterior by pores. This
longitudinal canal was lost in Invertebrates, but persisted in
Vertebrates as the pronephric duct, while the pores remained in
Invertebrates and disappeared in Vertebrates; (4) that the tracheae of
Arthropods, as well as the canal of the central nervous system in
Vertebrates, are to be traced back to certain ectodermal pits in the
diploblastic ancestor comparable to the sub-genital pits of the
Scyphomedusae. These ectodermal pits were all originally respiratory
organs. "The essence of all these propositions," he writes, "lies in the
fact that the segmented animals are traced back not to a triploblastic
unsegmented ancestor, but to a two-layered Coelenterate-like animal with
a pouched gut, the pouching having arisen as a result of the necessity
for an increase in the extent of the vegetative surfaces in a rapidly
enlarging animal (for circulation and respiration)" (p. 47). "I have
attempted to show," he writes further on, "that the majority of the
Triploblastica ... are built upon a common plan, and that that plan is
revealed by a careful examination of the anatomy of Coelenterata; that
all the most important organ-systems of these Triploblastica are found
in a rudimentary condition in the Coelenterata; and that all the
Triploblastica referred to must be traced back to a diploblastic
ancestor common to them and the Coelenterata" (p. 68). The main
assumption was that the neural or blastoporal surface must be homologous
throughout the Metazoa, though it was dorsal in the Chordata, ventral in
the Annelida and Arthropoda. He derived the central nervous system of
the Chordata from the circumoral ring of the common ancestor by means of
the hypothesis that both the pre-blastoporal and the post-blastoporal
parts of it disappeared.[451]

The characteristic relation of the central nervous system to the
blastopore in Annelida and Vertebrates had already been pointed out by
Kowalevsky,[452] who had also sketched a theory of the common descent of
these two phyla from an ancestral form in which the nervous system
encircled the blastopore.

In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put
forward the somewhat