310 ' J. W. KIRKALDY. 



The mid-ventral surface in front of the atriopore is traversed 

 by numerous longitudinal pleats, which permit of the expansion 

 and contraction of this area so as to alter very considerably the 

 volume of the viscera and cavities of the body. 



A cartilage-like tissue is developed as skeletal support for 

 the prseoral cirrhij and for the median and lateral (meta- 

 pleural) fin-plates. In the median dorsal fin, and usually, but 

 not always, in the median ventral fin, this tissue takes the form 

 of separate fin-rays, each contained in a lymph-space or fin-ray 

 chamber ; in the metapleura it forms a continuous unseg- 

 mented band with a related canal-like lymph-space. 



I find it desirable to recognise in the family Branchiosto- 

 midse two genera — Branchiostoma and Asymmetron. The 

 genus Branchiostoma is divided by me into sub-genera — Am- 

 phioxus and Heteropleuron. 



Genus I. — Branchiostoma. 



Branchiostoma, Costa. 1834. 

 Amphioxus, Yarrell. 1836. 



Prseoral tentacles or cirrhi, forming a single series united 

 to one another by a uniformly low intertentacular membrane. 



A median ventral tentaculum impar present, the rest sym- 

 metrical. 



Dorsal and ventral median^ fins expanded in the caudal 

 region to form a lancet-shaped so-called " caudal" fin, within 

 which the axis of the body terminates. 



Fin-ray chambers, with or without enclosed fin-rays, present 

 in the praeanal portion of the ventral fin. 



Infra-rostral fin continuous with the right metapleur; the 

 left metapleur dying out before reaching the infra-rostral fin. 



Atrial chamber produced behind the atriopore into a single 

 tapering caecum, reaching as far as the anus. 



Prseoral tentacles provided with numerous sensory papillae. 



* Although morphologically the ventral median fin is possibly a portion of 

 the right metapleur, it is convenient to retain the name "ventral median 

 fin " for that continuation of the right metapleuron which lies between atrio- 

 pore and tail terminus in the raid-ventral line. 



