214 



CHORDATA 



•^j 



/ 



■o 



>o 



Subphylum (Cephalochorda, Adelochorda, 

 or Acrania). Type — Branchio stoma ^ or 

 Amphioxus. — Amphioxus, the lancelet, is 

 transparent, less than three inches long, lives 

 in shallow sea water, partly buried in sand, 

 burrows head foremost, swims at night. 

 Beebe reports them in the Sargasso Sea. 

 (Figure loi.) 



It has a median dorsal fin, extending 

 posteriorly to form the caudal fin and then 

 ventrally to the post-atrial region to form 

 the ventral fin. The laterally situated meta- 

 pleural folds occupy the position of lateral 

 fins in fishes. 



Body Wall. — The outer covering is a 

 single layer of columnar epithelium cells, 

 the epidermis., with sensory cells, goblet cells 

 (unicellular glands) and ciliated cells. The 

 dermis consists of soft connective tissue. 



Muscular Layer. — The muscular layer 

 has strikingly definite metameric segmen- 

 tation. The myomeres, myotomes or muscle 

 plates are alternately arranged on the right 

 and the left sides permitting flexible lateral 

 movements and with connective tissue septa 



2 Balanoglossus, the Tunicates, and Branchiostoma 

 have the following characteristics in common at some 

 stage in their existence: (i) notochord; (2) pharyngeal 

 gill slits; (3) dorsal nervous system. 



Fig. ioi. A lateral view of Amphioxus (trans- 

 parency), a, anus; «./>., atrial pore; f./., caudal fin; 



^&D cir., cirri, on the edge of the vestibule leading to the 

 mouth; ^./., dorsal fin; r, fin rays; g, gill of branchial 

 structures consisting of alternate slits, through which the 

 water passes, and supporting plates, in the walls of 

 which are the blood vessels; in., intestine, from which 



: as a diverticulum springs /, the liver; m, the mouth sur- 



rounded by a fringed velum; my., myotomes or muscle 

 segments; n.c, notochord; 0, ovaries; s.c, spinal cord; 



'G ^•/•> ventral fin. (From Galloway. Courtesy of P. 

 Blakiston's Sons Co.) 



