354 C. FORSTER COOPER. 



4. PELAGIC LARVAE. PI. XVIII. figs. 3—6. 



Although larvae oi Amphioxiis were throughout found to be far from common, about a dozen 

 were taken in the south of Miladumadulu Atoll and in Fadifolu Atoll, during rough and 

 stormy weather in the last fortnight of December, 1899. They were caught at sundry depths 

 from 8 to 20 fathoms. All were secured during the night in the channels leading from the 

 open sea to the atoll lagoons by hanging tow-nets at different depths over the stern of our 

 schooner, in such a manner as to keep them well swung out by the currents, which drive 

 the various forms of the plankton into them. 



It cannot be ascertained with any certainty whether the larvae are those of Asymmetron, 

 Heteropleuron or of some other form the adult of which we did not obtain. There are many 

 reasons, derived from the anatomy of these larvae, why they should not belong to Asymmetron, 

 in spite of the fact that this form is present in our collections in the proportion of one 

 hundred or more to one of H. maldivense (the only other form found). 



The larvae evidently all belong to one and the same species, and, although they do not 

 form a series extending over any considerable period of development, seem to possess some 

 features worth describing. They are all in nearly the same stage, but owing to the unusual 

 nature of their gill clefts, cannot be compared at all closely in point of age with the well- 

 defined stages in the larval history of Amphioxus lanceolatus as described by Willey". 



In shape and external appearance the larvae are not unlike those of A. lanceolatus when 

 both are viewed from the side (PI. XVIII. figs. 3 and 4). There is a moderate-sized dorsal 

 fin extending from the tail to about the level of the end of the pharynx, in front narrowing 

 down to half its previous width, and continuing forward with the same breadth to the front 

 end of the body. The tail fin is lance-shaped, the dorsal and ventral halves being of precisely 

 the same depth. The ventral fin is a continuation of the tail fin, and is similar to, but 

 slightly deeper than, the corresponding part of the dorsal fin. It extends rather further forward 

 than the broad part of the latter, and passes to the right side of the anus. 



The youngest larva in the collection measures 5'75 mm. in length and 1'5 mm. in depth 

 (PL XVIII. fig. 3), and possesses 60 myotomes. Its external form is as described above, and 

 it has 14 gill slits on the ventral side open to the exterior throughout. A stout notochord 

 of tjqjical structure runs throughout the body from the tip of the snout to the extreme end of 

 the tail. 



The central nervous system commences in the second myotome, and extends to the extreme 

 end of the tail, the last part being very slender. The eye spot (Fig. 83) consists of a round 

 mass of black pigment situated on the upper wall of the brain cavity at a little distance 

 from its front end. It is of comparatively large size, and projects into the lumen of the 

 cavity in such a manner as to almost fill it. The eye thus differs a little in position and 

 shape from that found in A. lanceolatus, where it is a conical cap of pigment placed on 

 the front end of the brain. The anterior dorsal nerve is very conspicuous, and the spinal 

 nerves are given off in the usual manner. The olfactory pit is placed almost immediately 

 behind the eye spot. Except in being of a rather smaller size it in no way differs from that 

 of A. lanceolatus. The presence of this structure proves that these larvae cannot be considered 

 to be the larvae of the Maldive Asymmetron, in which it does not occur. 



^ Willey, Amphioxus and the Ancestry of the Vertebrates. 



