46 



ZOOLOGY 



SECT. 



aperture of considerable size, the alrioporc {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-anal 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 fore- 

 most, 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. 742) 



Tnyoi 



coeZ 



Fio. 742.— Ampliioxus lanceolatus. A, transverse section of the pharyngeal region, 

 a. dorsal aorta ; b. atrium ; c. notochord ; co. ccelome ; e. eudostyle ; g. gonad ; kb. hranchial 

 lamella;; kd. pharynx; I. liver; iny. niyon^er^4 Jt. nephridiuni ; r. neui'on ; in. spinal 

 nerves. B, transverse seetion of the intestinal region, atr. atriunl ; c.<£l. cujlome ; 

 d. ao. dorsal aorta ; iyit. intestine ; mt/oiii ; myomere ; nch. notochord ; neu. neuron ; s, int. v. 

 sub-intestinal vein. {A, from Hertwig, after Lankester and Boveri ; B, partly after 

 Bolph.) 



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

 are provided with sensory hairs. 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 epi- 

 dermis is the dermis, formed mainly of soft connective-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, separated from one another 

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

 appearance, in a surface view, of a series of very open Vs with 

 their apices directed forwards (Figs. 741 and 743). Each myomere 



