LESS, xxix GENERAL STRUCTURE 371 



A, longitudinal vertical section. 



B. horizontal section through the pharynx and gills, 

 c, transverse section through the trunk. 



The ectoderm is dotted, the nervous system finely dotted, the endo- 

 derm radially striated, the mesoderm evenly shaded, the coelomic 

 epithelium represented by a beaded line, and all skeletal structures 

 black. 



The body gives origin to the dorsal (D. F ] , D. F-), ventral ( V. F), 

 and caudal (C. F) fins ; the paired fins are not shown. 



The body-wall consists of deric epithelium (Der. Epthni), dermis 

 (Derm], and muscle (M) : the latter is metamerically segmented and is 

 very thick, especially dorsally, where it forms half the total vertical 

 height (C). 



The exoskeleton consists of calcified dermal spines (Derm. Sp) in the 

 dermis, and of dermal fin rays (Derm. F. .A 5 ) in the fins. 



The endoskeleton consists of a row of vertebral centra ( V. Cent} below 

 the spinal cord (Sp. Cd), giving rise to neural arches (N.A), which enclose 

 the cord, and in the caudal regions to hsemal arches (H. A] : a cranium 

 (Cr) enclosing the brain (Br) : upper and lower jaws : branchial arches 

 (Br. A] and rays (Br. A', Br. A"), shown only in B, supporting the 

 gills : shoulder (Sh. G) and pelvic (Pelv. G) girdles : and pterygiophores 

 (Ptgph) supporting the fins. 



The mouth (Mth) leads into the oral cavity (Or. cav), from which the 

 pharynx (Ph) and gullet (Gut) lead to the stomach (St) : this is con- 

 nected with a short intestine (Int) opening into a cloaca (Cl) which 

 communicates with the exterior by the vent (An). The oral cavity and 

 cloaca are the only parts of the canal lined by ectoderm. 



Connected with the enteric canal are the liver (Lr) with the gall- 

 bladder (G. Bl) and bile-duct (B. D}, the pancreas (Pn), and the spleen 

 (SpZ). The mouth is bounded above and below by teeth ( 7'). 



The respiratory organs consist of pouches (shown in B) communicating 

 with the pharynx by internal (/;//. br. ap) and with the exterior by 

 external (Ext. br. ap) branchial apertures, and lined by mucous mem- 

 brane raised into branchial filaments (Br. Fil). 



The heart (fft) is ventral and anterior, and is situated in a special 

 compartment of the coelome (Pcd). Six of the most important blood- 

 vessels, the dorsal vessel (dorsal aorta, D. Ao), the cardinal veins 

 (Card. F), the lateral vessels (lateral veins, Lat. F), and the ventral 

 vessel (intra-intestinal vein, /. int. V} are shown in c. 



The whole ccelome is lined by epithelium, showing parietal (Cxi. 

 Epthni) and visceral (Ccel. Epthni'} layers. 



The ovaries (Ovy) are connected with the dorsal body-wall : the 

 oviducts (Ovd) open anteriorly into the coelome (ovd'} and posteriorly 

 into the cloaca. 



The kidneys (K} are made up of nephridia (Nph) and open by ureters 

 ( Ur) into the cloaca. 



The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. Cav] 

 hollowed out in the dorsal body-wall : it consists of brain (Br) and 

 spinal cord (Sp. Cd), and contains a continuous cavity, the neuroccele 

 (;/. frt'). 



R B 2 



