Engraulis japonicus taken at random from the catch of commercial seine in the neighbouring waters of Ise Bay from 1951 to 1953 were examined, and the following knowledge was gained.
(1) Generally speaking, the number of the abdominal vertebrae (A. V. N.) is 26, and that of the caudal (C. V. N., excluding the urostyle) is 18 or 19, and the average number of the total vertebrae depends on the ratio of 18 to 19 in numbers of the caudal vertebrae in one batch of fish. (Table 1, 2 and 3)
(2) (a) In each of 983 fishes of 3, 854 samples (ca. 25%), one or two modified forms of vertebrae were encountered. (b) The vertebrae of modified form are classifiable into six types (Fig. 1, A-F), and these again can be grouped into two groupes-the one (A-C) has one or two additional spines and the other (D-F) has an incomplete spine or centrum. Fish belonging to the former group have 44 vertebral segments (18 caudal segments), and those of the later group have 45 vertebral segments (19 caudal segments). (Table 4, 5, 6 and 7). (c) Each type of modified vertebra has the fixed position, A is at L-1 (the last vertebra immediately preceding the urostyle), B, C and D at L-2 (the penultimate vertebra), E at L-3 (the pre-penultimate vertebra) and F at L-2 or L-3. (Table 8). (d) The fish with the modified vertebra may be considered as intermediate between the fish with 44 normal vertebrae and that with 45 normal vertebrae. (see Schema)
(3) In the tail part, neural and haemal spines are secured to the vertebrae by cross-ties (Fig. 1, Normal, L-1-L-4), and this part will be referred to as the “Tail Vertebrae”. The relation of the average number of the tail vertebrae (M. T. N.) to the average length of body (M. B. L.) is shown in Fig. 2.