SEA-FISHERIES LABORATORY, 231 



and intercrossing muscle fibres. The first diaphragm is 

 perforated by some of the retractors of the proboscis, and 

 it bears two backwardly projecting pear-shaped or finger- 

 shaped out-growths which lie to the sides of, and ventral 

 to, the oesophagus (fig. 23). These are not oesophageal 

 glands. They open anteriorly into the coelomic space in 

 front of the first septum. Their walls are muscular and 

 very vascular and they contract at frequent intervals 

 during life. Their function is unknown, they may pos- 

 sibly be of use in aiding the eversion of the proboscis. 



The occurrence of rudimentary septa accompanying 

 some of the afferent and efferent vessels of the nephridia 

 and gills, and the probable function of the two strands 

 (septa) attached to the efferent vessel of the fourth 

 nephridium have been described above (p. 228). 



The caudal septa are incomplete ventrally and ventro- 

 laterally, i.e., above and at the sides of the nerve cord. 

 Each is composed of two thin layers of coelomic epithe- 

 lium, forming the anterior and posterior faces between 

 which is a small quantity of connective tissue and muscle 

 fibres. These septa are not fenestrated like the three 

 anterior diaphragms. There are in the tail, in addition 

 to these septa, dorsal and ventral mesenteries, by means 

 of which the intestine is attached above and below to the 

 body wall and in which lie the dorsal and ventral blood 

 vessels. Owing to their small size these mesenteries are 

 with difficulty seen in dissections, but they are readily 

 distinguished in transverse sections of the tail. 



CCBLOM AND CcELOMIC FLUID. 



The coelom, which is lined by a layer of flattened 

 cells, is spacious and continuous from end to end of the 

 animal. It is partly sub-divided by septa in the anterior 



