40 



ZOOLOGY 



SECT. 



the ventral fin. The post-anal portion of the body is dis- 

 tinguished 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. 697) 

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



A T&- 



air 



FIG. 697. Amphioxus lanceolatus. A, transverse section of the pharyngeal region. 

 a, dorsal aorta ; b, atrium ; c, notochord ; co* ccelome ; e. endostyle ; g. gonad ; kb, branchial 

 lamellae ; M, pharynx r^i. liver; my. mypmere ; n, iiephridium ; /-, neuron; .-,>. spinal 

 nerves; sp. gill-slits. \T3, transverse section of the intestinal region, utr. atriuii 

 ccelome; d. uo. dorsal aorta'; int. intestine; mi/om. myomere ; nch. neuron ; 



s. int. v. sub-intestinal vein. (A, from'Hertwig, after Laiikester ai ; tly after 



Rolph.) 



are provided with sensory hairs. The epitheliurr 

 cirri presents at intervals regular groups of sensory >rne of 



them bearing stiff sensory hairs, others cilia. Eene- i-pi- 



dermis is the dermis, formed mainly of gelatinous connective 

 tissue. ' t 



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

 metameric segmentation. It consists of a large number abo it 

 sixty of muscle-segments or myomcres, separated from one another 

 by partitions of connective tissue, the myocomma*, and having the 

 appearance, in a surface view, of a series of very - w i \ h 



their apices directed forwards (Figs. 696 and 698). E; ch moT 



