xiii PHYLUM CHORDATA 41 



is composed of numerous flat, striated muscle plates, arranged longi- 

 tudinally, so that each is attached to two successive myo- 

 cornmas. In virtue of this arrangement the body can be bent 

 from side to side with great rapidity. The myomeres of the right 

 and left sides of the body are not opposite to one another, but 

 have an alternate arrangement. A special set of transverse muscles 

 (Fig. 697, 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- 

 cavity, 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. 697 and 698, 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 notochordal 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 notoclwrdal 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 resistent 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. 698, sk.) 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 chitinoid 

 material, the gill-rods (br. 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 Jin-rays (dors. f. r., vent. f. ?*.), short rods of connective tissue, 

 each contained in a cavity or lymph 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.). 



