IV STRUCTURE 107 
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, 1t 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 (coel), in which he the alimentary canal 
(int), the gonads (gon), 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) hes external to the coelom and body- 
wall around the pharynx and the greater part of the alimentary 
canal, and opens to the exterior by the atriopore. As in the 
Tunicata, the perforations (gill-slits) in the wall of the pharynx 
(6r.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 are 
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 
