xm PHYLUM CHORDATA 45 



(atrp.), and a short distance from the posterior extremity of the 

 body is the anus (an.), placed unsymmetrically on the left side of 

 the ventral fin. The post-anat portion of the body is distinguished 

 as the tail. 



Amphioxus ordinarily lives with the greater part of the body 

 buried in sand, only the anterior end with the expanded oral 

 hood protruding. It also swims in the vertical position, and 

 frequently lies on one side on the sand : it burrows, head foremost, 

 with great rapidity. A current of water is constantly passing in 

 at the mouth and out at the atriopore. 



Body-wall. The body is covered with an epidermis (Fig. 751) 

 formed of a single layer of columnar epithelial cells, some of which 

 are provided with sensory hairs, while some are unicellular glands. 

 On the surface of the epidermis is a cuticle perforated by pores. The 

 epithelium of the buccal cirri presents at intervals regular groups 

 of sensory cells, some of them bearing stiff sensory hairs, others cilia. 

 Beneath the epidermis is the dermis, formed mainly of soft connec- 

 tive-tissue. 



The muscular layer (my, myom.) is remarkable for exhibiting 

 metameric segmentation. It consists of a large number about 

 sixty of muscle-segments or myomeres s separated from one another 

 by partitions of dense connective-tissue, the myocommas, and having 

 the appearance, in a surface view, of a series of very open V's with 

 their apices directed forwards (Figs. 750 and 752). Each myomere 

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

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

 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. 

 751, 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. 751 and 752, c, nch.), a 

 cylindrical 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 

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



