520 THE ANIMAL KINGDOM. 



of the coelome is lined by an endothelium. The surface of the body is 

 covered with papillae. Closed capsules prolonged externally into a spine, 

 lined by hypodermis cells and supplied by a nerve, are found in the primary 

 papillae. They are very numerous on the antennae, lips, oral papillae, 

 and certain regions of the ventral surface of the feet, and are probably 

 tactile in function. The muscles, with the exception of those attached to 

 the jaws, are unstriped. 



The nervous system consists of a pair of supra-oesophageal ganglia 

 united medianly, of commissures surrounding the pharynx, and a pair of 

 widely separated ventral nerve-cords united dorsally and posteriorly by 

 a fibrous commissure above the anus. The two cords are united across 

 the middle ventral line by numerous transverse commissures. The an- 

 tennary nerves arise from the supra-oesophageal ganglia, those for the 

 jaws from the oesophageal commissures. The cords give off numerous 

 lateral nerves, those for the feet being especially stout. The transverse 

 commissures give off nerves to the skin. There is a ventral layer of ganglion 

 cells more especially aggregated at the origin of the nerves to the feet. 

 The sympathetic system consists of two nerves rising from the supra- 

 oesophageal ganglia and running dorsally on the walls of the pharynx 

 and oesophagus, where they unite. The eyes are paired, and lie dorso- 

 laterally, one at the base of each antenna. They are formed as invagina- 

 tions from the nervous thickenings of the prae-oral lobes of the embryo. 

 The vesicle thus formed lies just below the skin. Its anterior cells become 

 somewhat flattened, its lateral and especially its posterior cells elongated, 

 forming visual cells. These cells are pigmented at their outer ends ; at 

 their inner they are clear, i. e. bear retinidia, and touch the oval gelatinous 

 lens, which must be formed originally by secretion from the cells. There 

 is an optic ganglion. This eye resembles that of Chaetopoda and Gastro- 

 poda^. 



The digestive tract consists of the buccal depression above mentioned, 

 a muscular pharynx into which opens ventrally and posteriorly the common 

 duct of the two salivary glands, a narrow oesophagus, all lined by the 

 chitinoid cuticle ; of a wide mesenteron stretching nearly the whole length 

 of the body, and a short rectum lined by cuticle and opening by a terminal 

 anus. The rectum has an external circular and an internal longitudinal 

 layer of muscle fibres. The position of these layers is reversed in the 

 other sections of the tract A dorsal blood-vessel or heart in the shape 

 of a muscular tube with a pair of valved ostia to each somite of the body 

 lies in a pericardial cavity, closed below by a horizontal septum and placed 

 between the two dorsal muscle bands. It gives off no vessels. A delicate 

 ventral vessel lies externally to the circular layer of muscles. There is 



1 Patten considers that it represents the primitive eye of the Arthropoda, from which other forms 

 have been derived by abbreviation of development. 



