464 THE LANCELETS— CEPHALOCHORDA 



one of which occurs in Africa, another in South America, and 

 a third in Australia. We may take the Cephalochorda as an 

 illustration among marine animals. The meaning of the wide 

 representation of archaic types is, in part at least, that having 

 lived as a race for long ages they have had ample time and 

 many opportunities for wide dispersal. The fact in regard to 

 Lancelets is that they are found in relatively shallow water (rarely 

 below 50 fathoms) in all temperate, tropical, and sub-tropical 

 seas. Various species are reported from off the coasts of 

 Europe, North America, South America, Queensland, Japan, 

 Pacific Islands, Ceylon, Bahamas, etc. 



Lancelets feed on microscopic particles and organisms 

 which are swept into the mouth by the ciliated cirri. There is 

 no evidence of seeking or selecting food. Along the ventral 

 median line of the pharynx there is a glandular and ciliated 

 gutter, similar to the endostyle of Tunicates, and this secretes 

 mucus which is wafted forwards and entangles the food parti- 

 cles. Thin ropes of these pass backwards along a dorsal groove 

 to the digestive portion of the food-canal. The gut is ciliated 

 throughout, which is undoubtedly a primitive character. 



Water is always passing in by the mouth, through the 

 numerous slits in the pharynx into the atrial cavity, and out by 

 the atriopore. This current secures the aeration of the colour- 

 less blood in the delicate blood-vessels that run between the 

 slits. Thus the current of water has both a respiratory and a 

 nutritive significance just as in Tunicates. The current also 

 serves to waft out the ova and spermatozoa, for there are no 

 genital ducts. It is said that the current is sometimes reversed, 

 which explains how the sex-cells, which usually escape by the 

 atriopore, occasionally pass out by the mouth. 



The simple pouch or caecum which projects forwards at the 

 beginning of the digestive region of the gut is very interesting, 

 since it remains permanently at a level which represents the early 

 embryonic state of the liver in higher Chordates. No food is 

 found in the caecum ; it is a simple glandular diverticulum. 



The excretory tubules ornephridia which filter the blood 

 and get rid of nitrogenous waste products, are co-extensive 

 with the gill-clefts ; they are situated in the body-cavity and 

 open into the atrial cavity. There may be ninety pairs of 

 these nephridia which in general disposition and in some de- 



