THE LANCELET 335 



the originally continuous perivisceral coelom of the 

 pharyngeal region of the body, broken up into canals by 

 the piercing of the walls by the gill slits. Behind the 

 atrium, the perivisceral coelom is a narrow but unbroken 

 space and surrounds the alimentary canal, which runs 

 straight back through it to the anus. The part immediately 

 behind the pharynx is a very short, narrow oesophagus, 

 continuous with the epibranchial groove; then comes a 

 wider region known as the stomach. This gives off forward 

 on its right side a pouch known as the hepatic cacum or 

 liver, which pushes before it the atrial wall and lies in the 

 atrium beside the pharynx. Behind the stomach the gut 

 narrows and is known as the intestine. The whole alimen- 

 tary canal is lined by a ciliated columnar epithelium. 



In the dorsal wall of the atrium, lying between the 

 atrial epithelium and the dorsal coelomic canals, 

 organ*s. rV * s a ser ies of short tubular structures, corre- 

 sponding in number with the primary gill slits 

 and opening into the atrium opposite the dorsal ends of 

 the tongue bars. Each passes forwards round the top 

 of the" adjoining slit and gives off on its upper side 

 several short branches. From each branch there projects 

 into the ccelom a tuft of fibres known as solenocytes, which 

 are fine, protoplasmic tubules, each ending blindly in a knob 

 where there is a nucleus. The other end of the tubule 

 opens into one of the branches of the main tube. Each 

 tubule ; contains a long fiagellum, which arises from the 

 protoplasm round the nucleus and hangs into the main 

 duct. : These organs are known as nephridia, although 

 they do not, like the nephridia of the earthworm, open into 

 the coelom. Their solenocytes recall the flame cells of the 

 Platyhelminthes (p. 203). 1 It has been shown that if the 



: The relation between the excretory tubes of different animals is 

 very uncertain. Those of Platyhelminthes, the earthworm, and perhaps 

 Amphioxus arise in development from the ectoderm. Those of 

 Vertebrata, Mollusca, and perhaps the crayfish arise from the meso- 

 derm. The Malpighian tubes of insects belong to the ectodermal hind- 

 gut. It has been proposed to restrict the term nephridia to ectodermal 

 excretory tubes, whether they communicate with the coelom or not, and 

 to call the mesodermal tubes cmlomoducts . The generative ducts of the 

 earthworm, like those of molluscs, arthropods, and vertebrates, are 

 ccelomoducts, 



