. The Cambridge natural history. Zoology. BODY-CAVITIESâNERVOUS 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 variat
. The Cambridge natural history. Zoology. BODY-CAVITIESâNERVOUS 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, ), 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 '^'^â J,^::^JZ^L^ of the third body-cavities, closely invest DoUciwgiossus kmixdevskn the pharynx in some species. Body-Wall and Nervous System.â 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 ; , cir- cular nerve ; ), collar- pore ; d, dorsal nerve ;
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
