PISCES. 57 



organ ; this part of the central nervous system is represented by the 

 anterior obtuse extremity of the spinal cord. The chorda dorsalis, 

 terminating in a point at both ends, has two sheaths, from the most 

 external of which a fibrous lamina is extended upwards on each 

 side ; these two laminae approach each other and finally coalesce. 

 In this way a canal is formed above the chorda dorsalis in which 

 the spinal marrow is contained. But the entire canal is not filled 

 by this : it is divided by a thin transverse partition into a larger 

 inferior portion inclosing the spinal cord, and a smaller upper portion 

 containing a cellular tissue saturated with fat. Above this last 

 portion the two laminae unite to form a crest on which the rays of 

 the dorsal fin rest. The mouth is surrounded by two strips of 

 cartilage, consisting of jointed pieces, each of which terminates 

 laterally in a conical point ; these points are the supports of the 

 cirri that surround the mouth. Behind the mouth commences the 

 branchial cavity, which is supported by numerous cartilaginous 

 strips and invested by a nmcous membrane covered with vibratile 

 cilia ; small fissures are left between the strips of cartilage, which 

 do not open on the skin but terminate in the cavity of the body. 

 In front of the anus is an aperture (porus abdominalis) by which 

 the water escapes, and which may therefore be regarded as a 

 respiratory aperture, but through it the eggs and the sperma are 

 also evacuated. The intestinal canal, as in the Ascidice (of the 

 branchial sac of which Amphioxus reminds us), commences at the 

 bottom of the branchial cavity, and has a lateral expansion of a 

 green colour terminating in a blind extremity forwards, which 

 probably corresponds to the liver. The intestinal canal throughout 

 its entire extent is beset with vibratile cilia ; it has no convolutions, 

 and becomes narrower below j the anus is situated at a short dis- 

 tance from the extremity of the body, somewhat to the left side. 

 The sexual organs are in both sexes of the same structure, and 

 form on each side of the abdominal cavity a series of irregular four- 

 sided organs, which extends to the porus abdoniinalis, and are 

 visible through the skin. On the under surface, on each side, a fold 

 of skin has been observed, extending to the porus abdominalis, in 

 which a canal is situated terminating close to this last behind and 

 in front in the mouth. 



We give the preference to the name Amphioxus, although devised a 

 couple of years later than that of Branchiostoma for this genus of animals, 

 since the last is less commonly known, and moreover originated in the 

 mistake that the oral cirri are gills. 



