Fishes of the Western North Atlantic 3 



culiar adaptations for a very special mode of life. Perhaps the most that can be said at 

 present is that possibly they may be "fairly close to the primitive types from which the 

 vertebrates have arisen,'" although their atrial cavity has no parallel among the vertebrate 

 series.^ 



Order AMPHIOXI 



Description. This order includes all known representatives of the subphylum. They 

 are slender, fish-like in external appearance, the body tapers at both ends and varies in 

 length from one to eight cm. at maturity; they inhabit tropical and temperate seas. In the 

 adult the buccal cavity, which leads into the mouth proper, opens on the ventral surface of 

 the body a little behind the anterior end. It is bounded laterally by a pair of expanded 

 muscular membranes, the so-called oral hood, the free edge of which bears 20 to 30 

 slender oral tentacles or cirri, each supported by a cartilaginous rod arising from a 

 cartilaginous ring situated immediately behind the margin of the hood. Proximally, the 

 inner surface of the oral hood bears a series of finger-like projections of ciliated epithelium, 

 jointly forming the wheel organ, the ciliary action of which drives water inward through 

 the buccal cavity to the mouth, and so to the pharynx. The mouth, at the bottom of the 

 buccal cavity, is very small and surrounded by a vertical membrane, the so-called velum, 

 from which several short velar tentacles project inward into the capacious pharynx. The 

 linings of the pharynx, and of the vertical gill clefts that pierce its two sides, are clothed 

 with cilia (those of the former having a complex pattern), the joint action of which is to 

 drive the water from the mouth, along the pharynx, through the gill clefts and so out 

 through the atrial cavity and atriopore. The pharynx serves chiefly as a feeding organ, as 

 described below. 



The integument is expanded as a single continuous finfold which extends along the 

 ventral surface from close behind the atriopore, around the posterior end of the body, 

 thence forward along the dorsal surface and around the anterior end of the latter, where it 

 forms a snout or rostrum. The finfold thus surrounds the anterior end of the notochord and 

 contains a lymph space; in the dorsal fin this is segmentally divided by vertical septa into 

 a series of compartments known as fin-ray chambers and this is sometimes true of the 

 ventral fin as well. These chambers are partially subdivided by so-called fin rays, the 

 lateral and apical surfaces of which are free but the bases of which are connected with 

 the continuous ridge of connective tissue that is derived from the roof of the neural sheath. 

 The final number of rays and of ray chambers is established early in life, i.e., at a small size, 

 but is somewhat variable in all species. Anterior to the ventral fin the ventral surface of the 

 body also bears a pair of prominent longitudinal ridges called the metapleura. As a result 

 of their presence, the anterior part of the body is roughly triangular in cross section in 

 adults, the dorsal fin forming the apex of the triangle, the two metapleura its other two 



1. Romer, Man and Vert., 19+1 : 10. 



2. The atrium of the Lancelets, while analogous to that of the tunicates, cannot be regarded as homologous with the 

 latter, for the method of formation is very different. 



