326 MANUAL OF ZOOLOGY SECT. 



and Balanoglossus, is both a respiratory and a food current, the animal 

 feeding passively on the minute organisms in the surrounding water. 



There is a system of blood-vessels, but no heart. A contractile 

 median ventral vessel, the ventral aorta; runs forward in the ventral 

 wall of the pharynx, and gives off lateral branches, the afferent bran- 

 chial vessels, which pass upwards in the branchial lamellae. Efferent 

 branchial vessels receive the blood from the wall of the pharynx and 

 open dorsally into a pair of longitudinal vessels, the dorsal aortce. The 

 latter join to form a median dorsal aorta, which runs backwards 

 immediately below the notochord and above the intestine. 



The principal organs of excretion are about ninety pairs of peculiarly 

 modified nephridia (Fig. 191, iiph.) situated above the pharynx and in 

 relation with the main ccelomic cavities. An excretory function has 

 also been assigned to a single pair of organs called the brown funnels 

 (br. f.), also situated on the dorsal aspect of the pharynx at its 

 posterior end. 



The central nervous system is a rod-like organ, the neuron or dorsal 

 nei-ve-cord (sp. cd. ), contained within and completely filling a median 

 longitudinal netiral canal which lies immediately above the notochord. 

 It is traversed by an axial canal {cent. c. ) which becomes dilated at the 

 anterior extremity. From this nerve cord regularly (alternately) 

 arranged nerves are given off. 



At the level of the anterior end of the nerve-cord is a narrow ciliated 

 depression, the olfactory pit (olf. p.) opening externally on the left 

 side of the snout and connected at its lower end with a median hollow 

 process of the nerve cord. This structure is supposed to be an organ 

 of smell. 



The organ of sight is an unpaired //^w*/// spot (e) in the front wall of the 

 brain : it is therefore a median eye. A peculiar structure, the groove of 

 Hatschek, on the roof of the oral hood, is supposed to have a sensory 

 function, and may be an organ of taste. Lastly, the sensory cells on 

 the buccal cirri give those organs an important tactile function. 



The sexes are separate, but there is no distinction, apart from the 

 organs of reproduction, between male and female. The gonads (Fig. 

 190, gon.) are about twenty-six pairs of pouches arranged meta- 

 merically along the body-wall and projecting into the atrium so as 

 largely to fill up its cavity. 



When ripe the inner walls of the gonadic pouches burst, and the ova 

 or sperms make their way into the atrium and thence by the atriopore 

 to the exterior, where impregnation takes place. 



