XIII PHYLUM CHORDATA 47 



is coin])i)se(l of numerous flat, sti-iatod musclc-platcs, ari-angcd longi- 

 tudinally, so that each is attached to two successive myoconiinas. 

 In virtue of* this arrangement the body can be bent from side to 

 side with great rapidit}% The myomeres of the right and left 

 sides of the body are not oi)])osite to one another, but have an 

 alternate arrangement. A special set of transverse muscles (Fig. 

 742, A) extends across the ventral surface of the anterior two- 

 thirds of the body, lying in the floor of the atrial cavity presently 

 to be described. 



One striking and characteristic feature of the muscular layer of 

 the body-wall is the immense thickness of its dorsal portion. In 

 the higher Worms and many other Invertebrates the muscles form 

 a layer of approximately equal thickness surrounding the body- 

 cavit}', which contains, amongst other organs, the central nervous 

 system. In Vertebrates, on the other hand, the dorsal body-wall 

 is greatly thickened, and in it are contained both the nervous 

 system and the notochord. 



Skeleton. — The chief of the skeletal or supporting structures 

 of the Lancelet is the notochord (Figs. 742 and 743, f, nch.), a cylin- 

 drical rod, pointed at both ends, and extending from the anterior 

 to the posterior end of the body in the median plane. It lies 

 immediately above the enteric tract and between the right and 

 left myomeres. It is composed of a peculiar form of cellular 

 tissue known as notochorclal tissue, formed of large vacuolated 

 cells extending from side to side of the notochord, and having 

 the nuclei confined to its dorsal and ventral regions. Around 

 these cells is a notocliorclal sheath of connective-tissue, which is 

 produced dorsally into a canal for the nervous system. The noto- 

 chord, like the parenchyma of plants, owes its resistant character 

 to the vacuoles of its component cells being tensely filled with 

 fluid, a condition of turgescence being thus produced. 



The oral hood is supported by a ring (Fig. 743, sh.) of carti- 

 laginous consistency, made up of separate rod like pieces arranged 

 end to end, and corresponding in number with the cirri. Each 

 piece sends an offshoot into the cirrus to which it is related, 

 furnishing it with a skeletal axis. 



The pharynx is supported by delicate oblique rods of a firm, 

 gelatinous material, the gill-rods (hr. r.). These will be most 

 conveniently discussed in connection with the pharynx itself. The 

 dorsal fin is supported by a single series, and the ventral fin by a 

 double series of fin-rays {dors. f. r., vent. f. r.), short rods of con- 

 •uecti\e-tissue, each contained in a cavity^ or lym^jh-space. 



Digestive and Respiratory Organs. — The mouth (mth.), as 

 already mentioned, lies at the bottom of the vestibule or cavity of 

 the oral hood (or. hd.). It is a small circular aperture surrounded 

 by a membrane, the velum (vl.), which acts as a sphincter, and 

 has its free edge produced into a number of velar tentacles {vl. t.). 



