1 76 COMPARATIVE ANATOMY 



necessary that the relations of the atrium should be thoroughly 

 understood. On making a median ventral incision from the 

 atriopore to the vestibule one exposes a cavity which appears 

 to be traversed by the gut, and to contain the gonads, the 

 latter organs having the form of twenty-six pairs of pouches, 

 extending from the tenth to the thirty-sixth myotomes, and 

 projecting into the atrial cavity. This cavity appears to be, 

 and was long mistaken for, the perivisceral cavity or ccelom. 

 But in reality it is external to the body, and is lined with 

 epiblast. The gonads, though they seem to lie in it, are 

 really outside of and only project into it. The relations of the 

 atrial chamber in the adult animal are best understood by 

 imagining the side walls of the body of a fish-shaped animal to 

 be very extensible. If these side walls were pulled out on 

 either side of the body as a longitudinal fold, and the two 

 folds were drawn down below the ventral surface and fastened 

 together in the middle line like the flaps of a coat, there 

 would be a space between the wall of the body and the united 

 flaps which would correspond exactly in position to the atrial 

 cavity. One may realise the relations even more clearly by 

 imagining the sides and back of one's waistcoat to be part of 

 and continuous with one's skin. Then the space between the 

 front of the waistcoat and the body would correspond to the 

 atrial cavity, and the opening below the waistcoat would 

 correspond to the atriopore. It must be remembered, how- 

 ever, that though this illustration serves very well to explain 

 the adult relations, the atrial chamber is not actually formed in 

 Amphioxus by the downgrowth and union below of two flaps 

 of skin, but in a more complicated and curious manner which 

 will be described later. For present purposes we may speak 

 of right and left atrial folds, united in the mid-ventral line. 

 The ventral portions of the folds lying between the metapleurs 

 and the ventral suture are known as the epipleurs, the meta- 

 pleurs being, as it were, lateral offsets of the atrial folds. The 

 atrial chamber is closed in anteriorly, and has no communica- 

 tion with the vestibule in front of it. 



The cirrhi which surround the buccal folds are covered 

 with patches of sensory epithelium provided with stiff hair-like 

 processes, or with cilia, and are supported by skeletal rods 

 attached to a jointed skeletal hoop which runs round the 

 margins of the buccal folds. Internally the epithelium of the 



