348 



C. FORSTER COOPER. 



1. ASYMMETRON LUCAYANUM (Andrews). PI. XVIII. fig. 1. 



This species of Cephalochorda is by far the most abundant in the Maldives, occurring 

 roughly in the proportion of 100 to 1 of the only other form found (Heteropleuron maldivense). 

 It seems to be identical in almost every particular with the form described by Andrews from 

 the Bahamas'. 



The only point in which the Maldivan and W. Indian forms consistently differ from 

 one another is in their size. The average length of the Maldivan specimens is 23 mm., the 

 extremes being 18 and 30 mm., thus being nearly double the length of the Bahama specimens 

 which Andrews found to average 13 mm. In spite of this difference the average myotome 

 formula for the two forms remains practically the same, the mode in each case being 66 myotomes, 

 i.e. 44 from the head to the atriopore, 9 from the atriopore to the anus, and 13 from the anus 

 to the tail. 



The most prominent external feature of the species is the long caudal extremity which 

 at once differentiates it from all other known forms of Cephalochorda. There is a narrow 

 rostral fin round the anterior end of the notochord, which projects some 2 mm. beyond the 

 end of the first myotome. This is continued dorsally as a low fin running the whole length 

 of the body to the tip of the caudal extremity. Owing to the taper of the body the fin 

 is rather deeper for the last fifteen myotomes {i.e. from about the level of the anus to the 

 caudal extremity). From the last myotome to the tip of the tail it narrows down to a point. 



The ventral fin is a continuation of the right metapleural fold, and like the dorsal fin it 

 also broadens out for the last fifteen myotomes, though somewhat deeper than the last named. It 

 bends up suddenly at the last myotome, and becomes the ventral part of the narrow tapering 

 caudal fin. The dej^th of the dorsal and ventral halves of the latter fin is the same. 



In the dorsal fin from about the 17th to the 32nd myotome there are fin-ray spaces 

 containing small blocks of cartilage, the fin rays. These begin anteriorly as small irregular 

 blocks, which gradually increase in size and become 

 more regular, averaging two to a myotome ; towards 

 the tail they again decrease in size. There is, 

 however, much variation in different specimens 

 both in the number and size of the fin rays. 

 The ventral fin possesses neither fin rays nor fin- 

 ray spaces. 



In comparison with Amphioxus lanceolatus the 

 general shape of the species is long, thin, and 

 somewhat narrow from side to side. The meta- 

 pleural folds end short of the buccal hood, and do 

 not in any way connect up with it. The buccal 

 hood is long and shovel-shaped, but not very deep. 

 The buccal cirri number 19, and are divided into 



two groups on each side of a median unpaired tentacle. The outer group on either side 

 consists of five tentacles joined together by a low membrane, whilst each inner group jiossesses 

 four tentacles connected by a high membrane which reaches fully three-quarters of the way 

 up the cirri (Fig. 76). 



' Stud. Johns Hopkins Biol. Lab. vol. v. 



Fig. 76. Buccal cirri of Asymmetron lucayanuin. 



