84 



COMPARATIVE ANATOMY 



An emliyo of Amphioxus, at a stage when fourteen or fifteen pairs of 

 mesodermal pouches are present, is a delicate colorless transparent 

 animal having a length of about one millimeter and a diameter of one- 

 eighth that except at the somewhat enlarged head end (Fig. 57). It has 

 a straight digestive tube (enteron) /, extending from an anterior mouth to 

 a posterior anus. There is a single gill cleft, opening from the right side 

 of the anterior region of the digestive tube. The mouth also is unsym- 

 metrical at this stage, opening on the left side. Later, as numerous 

 additional gill clefts are formed, they shift their positions so as to become 

 ultimately a series of symmetrically placed paired apertures. Meanwhile 

 the mouth shifts from its original left to a median position. Just above 

 the digestive tube lies the median rod-like notochord {NC) extending the 

 entire length of the animal. Immediately above the notochord is the 



Fig. 57. — Amphio.xus at beginning of larval period; 14 or 15 pairs of mesodermal 

 somites. Actual length of larva about i.o mm. CG, club-shaped gland; /, intestine; 

 MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore; 

 NT, neural tube; P, pigment spot in neural tube. (After Hatschek.) 



neural tube (NT), its somewhat enlarged anterior region suggesting a brain. 

 At the anterior end of the neural tube the dorsal neuropore (NP) is still 

 open. The neurenteric canal (NE), at this stage, has ordinarily become 

 closed. In the anterior region, where the differentiation of the mesoderm 

 is most advanced, a coelom intervenes between the enteric tube and the 

 outer body- wall (Fig. 56, C). The body wall (somatopleure) consists of 

 the ectoderm and the somatic layer of mesoderm. The enteric endoderm 

 together with the contiguous visceral or splanchnic layer of mesoderm 

 constitute the wall (splanchnopleure) of the digestive tube. The somatic 

 and visceral sheets of mesoderm provide the coelom with a continuous 

 and complete lining, the peritoneum. The superficial ectoderm is a skin. 

 The more anterior myotomes contain partially differentiated muscle tissue 

 capable of feeble contraction. The animal is free-swimming but the 

 locomotor mechanism consists merely of long cilia produced by the ecto- 

 dermal layer. 



In its main features this young Amphioxus is like a vertebrate. If 

 its true origin and nature were not known, it might reasonably be expected 

 to proceed to develop directly into a typical vertebrate. But it does not. 

 It acquires no vertebral column; the notochord serves as definitive axial 



