CHAPTER III.
SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.
SUBPHYLUM A. HEMICHORDATA.
The subphylum includes three genera, Balanoglossus[19], Cephalodiscus and Rhabdopleura; and perhaps a fourth, Phoronis.
The skeletal structures found in Balanoglossus[20] are all endoskeletal. They include:
(1) The notochord. This arises as a diverticulum from the alimentary canal which grows forwards into the proboscis and extends beyond the front end of the central nervous system. It is hypoblastic in origin and arises in the same way as does the notochord of Amphioxus. Its cells become highly vacuolated and take on the typical notochordal structure[21]. The cavity of the primitive diverticulum becomes obliterated in front, but behind it opens throughout life into the alimentary canal.
(2) The axial skeletal rods. These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass.
(3) The branchial skeleton. The gill bars separating the gill slits from one another are strengthened by chitinous rods in a way closely similar to that in Amphioxus. But between one primary forked rod and the next there are two secondary unforked rods—not one, as in Amphioxus.
(4) The chondroid tissue. This is of mesoblastic origin and may be regarded as an imperfect sheath for the notochord.