BODY-CAVITIES — NERVOUS SYSTEM 



The hinder part of the proboscis-cavity is divided by the 

 forward growth of the notochord (Fig. 3, n) into dorsal and ventral 

 portions. The dorsal cavity in extending backwards liecomes 

 further subdivided into right and left 

 halves, the latter typically opening dors- 

 ally to the exterior on the proboscis- 

 stalk by an asymmetrical " proboscis-pore " 

 (p-P-)- Two symmetrical proboscis-pores 

 may, however, occur, or a median pore 

 connected with the left division of the 

 proboscis-cavity. These may be indi- 

 vidual variations within the limits of a 

 single species, or may occur as a normal 

 feature of a species. 



The collar-cavities open by two " collar- 



PP --!( 



TVS 



Cf-, 



pores" (Fig. 3, c.p.), situated at the 

 posterior end of the collar, into the first 

 pair of gill-pouches, near their external 

 opening. Willey has recently described^ 

 vestigial pores in relation with the " peri- 

 haemal spaces," a pair of dorsally situated 

 outgrowths of the third body-cavities 

 into the collar-region. Narrow " peri- 

 pharyngeal spaces," also a forward growth '^'^■J,^::^JZ^L^ 

 of the third body-cavities, closely invest DoUciwgiossus kmixdevskn 

 the pharynx in some species. 



Body-Wall and Nervous System. — 

 The body-wall (Fig. 4) consists externally 

 of a thick ciliated epidermis (e), con- 

 taining numerous gland-cells which secrete 

 an abundant mucus. Beneath the epi- 

 dermis is a basement-membrane, while 

 more internally are layers of muscles, 

 whose arrangement differs in different 

 parts of the body and in different species. 



The nervous system consists of a plexus of cells and fibres 

 which lie in the basal part of the epidermis of all parts of the 

 animal, outside the basement-membrane ; the thicker portions of 

 the plexus forming definite nerve-tracts. This intimate connexion 



1 Zool. Res. iii. 1899, pp. 273, 280. 



^V 



X 3. c, Collar ; c.n, cir- 

 cular nerve ; c.j), collar- 

 pore ; d, dorsal nerve ; g, 

 gill-pore ; «, notoeliord ; 

 n.s, central nervous sys- 

 tem, showing the anterior 

 and posterior neuropores ; 

 ]}, proboscis ; p.p, probos- 

 cis-pore ; p.s, proboscis- 

 stalk ; t, trunk ; v, ventral 

 nerve. The nerve-plexus 

 of the proboscis is repre- 

 sented as a black line. 

 (After Bateson. ) 



