STRUCTURE I I 7 



placed on the left side of the ventral fin, some distance behind 

 the atriopore, and not far from the posterior end of the body. 

 The short region behind the anus and surrounded by the caudal 

 fin may properly be called " tail." The current of water for 

 respiratory and nutritive purposes, and which may carry the ova 

 and spermatozoa to the exterior, usually passes in at the mouth 

 and out at the atriopore, as in the Tunicata. On occasions, 

 however, it is said to be reversed. 



General Structure. — The general plan of organisation of 

 the body (see Fig. 71) is that a longitudinal skeletal axis, the 

 notochord (nch), separates a dorsal nervous system (sp.cd) from a 

 ventral reduced coelom {cod), in which lie the alimentary canal 

 {int), the gonads (j/on), and other organs. Thus a transverse 

 section of the body (see Fig. 72) shows the typical Chordate 

 arrangement of parts, and is comparable with a transverse section 

 of a tadpole, a young fish, or a larval Ascidian. A peribranchial 

 (atr) or atrial cavity (which is morphologically a part of the 

 external world shut in) lies external to the coelom and body- 

 wall around the pharynx and the greater part of the alimentary 

 canal, an^_ opens to the exterior by the atriopore. As in the 

 Tunicata, the perforations (gill-slits) in the wall of the pharynx 

 (br.cl) open into the atrial cavity and so indirectly to the 

 exterior. 



Musculature. — The thick body -wall is largely formed by 

 muscular tissue metamerically segmented into about 60 myotomes 

 (Fig. 71, myom). These muscle-masses, which (as is usual in 

 Vertebrata) are thickest dorsally at the sides of the notochord 

 and spinal chord (Fig. 72, m), are so arranged as to present the 

 appearance in a lateral view of the body of a series of shallow 

 cones (« ) fitting into one another and with their apices 

 directed forwards. The muscle fibres are striated, and run longi- 

 tudinally along the body from the anterior to the posterior edge 

 of each myotome, so as to be attached at their ends to the two 

 septa of connective tissue which form the boundaries of the 

 myotomes. These septa, the myocommas, are conspicuous features 

 in the external appearance of the body (Fig. 70, B). They aTe 

 not arranged so- as to be opposite one another on the two sides, 

 but the myotomes on the right and left sides alternate, as can be 

 seen in a transverse section (Fig. 74, A, p. 121). It is by 

 means of these lateral muscle-bundles that the rapid vibration 



