1880. | Development of Amphioxus lanceolatus. 75 
much in shape and appearance. The last pair of nerves which arise 
at some distance from the end of the chorda, are directed back- 
wards and perform the same offices for the posterior end of the 
notochord and the tail, that the first pair do for the opposite 
extremity, but the nerves themselves are not so large, being 
slightly larger than the ordinary body nerves, and they do not 
divide into nearly as many branches. 
Muscular System-—The muscular system may be considered as 
made up of two sets of body muscles; two sets of transverse 
abdominal muscles ; few or many longitudinal fibres in and along 
the abdominal walls; mouth muscles; pharyngeal ring muscles, 
and sphincters for the mouth, branchiopore and perhaps for the 
ring or neck of the pharynx. 
The ody muscles are disposed as in ordinary fishes, one set 
upon either side and each set is composed of a series of regular, 
overlapping muscle plates, which extend from near the anterior 
extremity of the chorda dorsalis to within a short distance of its - 
posterior termination. Along the entire length of their dorsal half, 
the two sets are either united or very closely approximated, and 
enclose the greater portion of the entire length of the chorda and 
of the notochord, but leave the extremities of both entirely bare 
of muscular covering (Fig. 1, Pl. 1). These muscles are also 
united along the ventral edge e the tail portion, but for the ante- 
rior two-thirds or more of their length their ventral edges are 
quite widely separated, spreading outward and downward from 
the notochord to form the upper portion of the parietes of' 
the abdomen. The general outline of these combined plates, 
when seen from the side, is that of a long, slender spindle, 
gradually tapering from the center to a sharply pointed termi- 
nation at either end. Each plate is composed of longitudinal, : 
striped, muscular fibres, and is four-sided in outline, but with 
_ outline, tong and narrow, and with the longer points coincides 
direction with the notochord. Nearer the center of the body th 
plates are almost square, the- diameters being nearly equal ; 
_ while along the middle third, where they become once more 
_ Thomboidal, or nearly so, the longer diameter is at right angles to 
a the notochord, with the longer portion of each plate running 
o down into the abdominal walls, On account of the avecloai 
S each T T the one inmate ee to it, this 
