590 THE ANIMAL KINGDOM. 



or pouches. The cavities in the trunk are completely closed, but large. 

 They contain a fluid, in which, when coagulated by reagents, Spengel ob- 

 served stellate (? amoeboid) cells. 



The nervous system consists of a dorsal and ventral cord, extending 

 to the anus, and connected by a pair of ring-like thickenings in the collar, 

 and of a delicate network of fibrils beneath the epidermis. The cords are 

 sub-epidermic except in the region of the collar, where the dorsal cord is 

 completely independent of the epidermis. The deep part of the cord is 

 fibrous, the superficial part cellular. The dorsal cord in the collar contains 

 a central lumen produced by invagination in B. K&waUwskii (Bateson), or 

 either globular or long irregular cavities, lined by a cuticula (Spengel), 

 found also in the middle and posterior regions of adults (Bateson). The 

 dorsal cord is continuous at its anterior end, with a thick circular fibrous 

 layer at the base of the proboscis, which thins away anteriorly. Ganglion 

 cells occur at rare intervals in the cords, except in the dorsal cord of the 

 collar, where they are numerous. Hence this part may be regarded as the 

 central nerve-ganglion (Spengel). Organs of special sense are absent. 



The digestive tract is straight. At its anterior end it sends forwards 

 into the proboscis a diverticulum. The cavity of this process is obliterated 

 anteriorly by cells which are vacuolated with nodal nuclei, thus resembling 

 notochordal tissue (Bateson). On its ventral surface is formed a chitinous 

 bar, prolonged behind into two rods, which lie in the walls of the digestive 

 canal in the collar, and give attachment to longitudinal retractor muscles. 

 The first section of the digestive tract is incompletely divided into a dorsal 

 respiratory, and a ventral alimentary part, by two lateral longitudinal 

 folds. The respiratory portion gives origin to paired gill-pouches. These 

 pouches open externally on each side by small pores situated in a longitu- 

 dinal dorso-lateral furrow : internally, by long U-shaped slits. New gill- 

 pouches appear to be constantly added throughout life at the hind end of 

 the series. They are at first circular, like the gill-pouches of the embryo ; 

 but attain the characteristic U-shape by the downgrowth of a dorsal pro- 

 cess or valve. The sides of the pouches, and the partitions between them, 

 are supported by chitinous lamellae. Their margins are ciliated, and 

 water passes through them from within outwards. They receive their 

 blood from the median dorsal vessel. The section of the digestive tract 

 behind the branchial region is traversed by a longitudinal dorsal and 

 ventral furrow richly ciliated. In some species (B. minutus, B. claviger, 

 &c.), the middle section of the tract possesses paired dorsal liver-caeca, 

 the cells of which contain a brown or green pigment. These caeca pass 

 outwards into lobes of the body-wall, and possess an external aperture 

 in B. Salmoneus. For the dorsal a'nd ventral mesentery see supra. The 

 latter is lost in the collar ; as also is the dorsal mesentery, except in 

 B. Kowalewskii and B. Salmoneus, 



