138 



CEPHALOCHORDATA 



Thus sixteen species have been described, of which the three 

 under Br ancliio stoma placed after square brackets, seem to be 

 merely varieties of B. lanceolatum, and B. ncikagawae is probably 

 identical with B. helcheri ; while it is a question whether Asym- 

 metron caudatwn is more than a variety of A. lucayammn, thus 

 leaving eleven or twelve species that seem fairly well character- 

 ised. The exact positions of the two marked (?), viz. B. elonga- 

 tum, and B. pelagicum, cannot be determined in the absence of 

 fuller descriptions of these species. 



40 80 



120 160 



3iayi/srcCs Gay^ £slui' londun. 



Fig. 90. — Sketch-map showing geogi'aphical distribution of the Cephalochordata. 

 + indicates Branchiostoma ; o indicates Asyvimetron. 



The list above, and the map (Fig. 90), give some indication 

 of the geographical distribution of the group, and show that, 

 although the few species are widely distributed over the shallow 

 waters of the globe, most of the records lie between 40""^. and 

 40° S. latitudes. In fact the group is mainly a tropical one, 

 and is most abundant in the Indo-Pacific region. The crosses 

 indicate records of species of Branchiostoma, and the circles those 

 of Asymmetron (including Heterofleuron) ; the latter are confined 

 to the Indo-Pacifio seas, with the exception of A. lucayanum 

 from the Bahamas — one of the numerous cases of interesting 

 similarity between the marine faunas of the East and West 

 Indies. 



