AN UNDESCEIBED ACEANIATE. 219 



intestine show the first cleaveage spindles, having been fertilized. 

 As all these cases were specimens kept in confinement we infer that 

 these sexual products were probably swallowed and give no evidence 

 as to the method of discharge from the gonads. None were found 

 free in the atrium ; and in some cases the individual with ova in 

 its digestive tract was sexually immature. 



Even more conspicuous than this asymmetry of the reproductive 

 organs is the character of the fins at the posterior end of the animal. 



As seen in Figs. 1, 2, 4, there is a long slender caudal process 

 extending posterior to the last myotome. This process is composed 

 of the rather low dorsal and ventral median fins which become 

 continuous at its tip but are elsewhere separated by the supporting 

 notochord that extends nearly to the very tip of the caudal pro- 

 cess. Moreover the nerve cord runs back in this caudal process 

 as a slender tube just dorsal to the notochord and finally diminish- 

 ing to a few cells surrounding a lumen at the extreme tip of the 

 process. In a transverse section (Fig. 23) taken about the region 

 h of Fig. 4 the dorsal and ventral fins are almost identical in size 

 and structure. Each is a clear matrix of connective tissue perme- 

 ated by irregularly radiating, anastomosing canals lined by cells 

 that may nearly fill them, as represented to the extreme right in 

 Fig. 25. This clear substance is concentrated as a sheath about 

 the notochord and serves peripherally as a support for the single 

 layer of epidermal cells covering the caudal process as all other 

 parts of the body. The minute nerve cord has ganglion cells but 

 no pigment. The numerous nerves branching through the caudal 

 process come for the most part, if not altogether, from the neural 

 tube anterior to the caudal process. 



In the youngest forms found, having a length of 6 mm. and 

 22 branchial clefts on a side, the caudal process is very much 

 shorter, not narrowed, but on the contrary expanded as the charac- 

 teristic, rounded, larval tail-fin (Fig. 5). Soon, however, this 

 rounded tail-fin becomes pointed, as seen in a specimen with twenty- 

 seven branchial slits (Fig. 3), and subsequently elongated as in the 

 sexually mature individuals. 



Where the caudal process springs from the myotome region the 

 ventral and the dorsal median fins both suddenly increase in height, 

 the ventral much more than the dorsal. 



