STRUCTURE OF AMPHIOXUS. 365 



Three apertures are evident ; (a) the mouth fringed by little 

 tentacle-like processes, and overarched by a pre-oral hood ; (b) 

 about two-thirds of the animal's length from the head the 

 atriopore or branchial pore (in myotome thirty-six), through 

 which the water which enters by the mouth and washes the 

 gill-slits passes out ; (<r) the anus nearer the posterior end of 

 the body. On each side of the region between the mbuth 

 and the branchial pore, are two folds or epipleura, which 

 have covered over the originally external apertures of the 

 gill-slits, and have formed an atrial or branchial chamber. 

 This region in front of the atrial pore is marked externally 

 by its flat ventral surface, which is fringed on each side by 

 a fin-like " metapleural" fold. These are due to the epipleural 

 flaps mentioned above. As will be afterwards seen, the 

 animal is less perfectly symmetrical than it at first sight 

 appears, and perhaps this is due to its habit of frequently 

 lying on one side. 



Skin. — The epidermis is a single layer of cells, some 

 of which, with projecting tips and basal connections with 

 nerve-fibres, are sensory. Very similar cells occur along 

 definite lines in fishes, and also on young tadpoles. Beneath 

 the epidermis lies a clear cutis without nuclei. Beneath this 

 are fine tubes which seem to be derived from the body- 

 cavity, and unite in a longitudinal canal running along each 

 metapleural fold. The median fin extends along the back, 

 around the tail, and between the tail and the atriopore. The 

 dorsal and the ventral fins are supported by what may be 

 called "fin-rays''; of these there are 250 along the back. 



Skeleton. — This is very slightly developed. There is not 

 only no bone, but the supporting material is not even 

 definitely cartilaginous. 



(a) The elastic no'tochord runs from tip to tip. It is 

 unsegmented, and is surrounded by a c9ntinuous sheath of 

 connective tissue. 



(b) The respiratory pharynx is supported by a system of 

 horny (?) rods, which lie between the numerous gill-slits. 

 There is also a paired longitudinal plate along the mid- 

 ventral groove of the pharynx. 



(c) The mouth is embraced by two curved bars, each 

 segmented into about a dozen pieces, which bear filaments- 

 supporting the tentacles. 



