Developmental Changes in Small Bottom Forms. 17 



All the specimens were measured after preservation in formalin. The Angler 

 is a form on which measurements are not easily taken, but, as far as possible, only 

 those have been selected which can be made between definite and comparable 

 points in the several specimens. For the present purpose, the specimens have been 

 arranged in series according to their lengths from the tip of the lower jaw to the 

 end of the tail peduncle [i.e., length without caudal fin). For each specimen, 

 this measurement has been used as the standard, and all other measurements have 

 been calculated as percentages of this standard length. The total length has been 

 given in brackets for the purpose of ready reference. 



The head length of Lophius is difficult to define, and measurements have there- 

 fore been made from both the lower jaw and upper jaw to a point in the occipital 

 region between the two strong occipital spines. Breadth of head has been measured 

 behind the eyes, the fringe of tag-like processes acting as a guide to the limits. 

 Depth of head is perhaps the most unsatisfactory measurement, as it depends 

 greatly on the degree of distension or depression of the head at the time of death. 

 The vertical distance of the occiput above the flat surface on which the specimen 

 lay has been taken as a measure. In order to secure uniformity for the paired 

 fins, these have been measured from their insertions to the tip of the longest ray. 



The number of specimens examined is certainly small, and discrepancies may 

 arise through individual variations, but, when these points are taken into con- 

 sideration, the general trend of the develoj^mental changes is sufficiently obvious 

 to allow certain conclusions to be drawn from the data obtained. The results - 

 have been plotted out in a diagram, the standard lengths being represented as 

 abscissae, the percentage-lengths of the various measurements as ordinates. Any 

 change in the relative growth of the different parts with increasing size of specimen 

 will thus be indicated by the slope of the fines. Curves have been drawn as uniformly 

 as possible among the points for each series of observations. After a length of about 

 150 mm. (185 mm.), these curves in general run parallel to the abscissal axis. The 

 interpretation is that the young Angler at a length, without caudal fin, of 150 mm. 

 (or about 185 mm. total length), has completed its metamorphosis, and acquired 

 its true bottom form, and that thereafter the growth of the different parts is uni- 

 form. 



The changes from the typical pelagic form to the bottom form of 150 mm. length 

 (185 mm.) are orderly and progressive. The length, breadth, and depth of the head 

 decrease regularly, although slightly, in proportion to the length of the fish. The 

 relation, head length to head breadth, is established before the Angler reaches a 

 length of 48 mm. (or 64 mm. total length). On the other hand, the distance from 

 the tip of the lower jaw to the anus, relative to the total length, increases sHghtly 

 as the fish grows, until the length of 150 mm. (185 mm.) is reached. In other 

 words, until this length is reached, the body portion between the head and anus 

 grows relatively most quickly. 



There are also orderly changes in the relations of the fins to the total body 

 length. Here again, however, the paired fins and the caudal fin have assumed 

 their definite proportional size when the fish has reached a length of about 150 

 mm. (185 mm.), and after this length, these fins grow uniformly with the fish. 

 The transition is most rapid in the relative length of the pelvic fins, and least rapid 

 in the caudal fin, as was to be expected. The proportional length of the pectoral 

 fins to the total length falls from over 50 per cent, at a length of 48 mm. (64 mm.) 

 to an average of 33 per cent, at 160 mm. (195 mm.), whilst the pelvics fall from 

 40 p.n- cent, to between 16 and 17 per cent, within the same limiting sizes. 



The changes in growth of the rays of the first dorsal fin are also of considerable 

 interest. It has already been seen that the first dorsal fin, on account of the ex- 

 tension of the rays and the development of the membranous sheath is of consider- 

 able importance during pelagic Ufe. The first two rays, however, are not included 

 in this common membranous sheath, but are gradually shunted further and further 

 forward on the top of the head by the rapid growth of the interspinous elements, 

 until their free portions arise only a very short distance behind the tip of the snout. 

 The first ray is the last to develop, and is at first very much smaller than the second 

 ray. In the smallest bottom form examined, it is less than half the length of the 

 second ray. The diagram shows that the second ray loses its relative importance, 

 and only after the small bottom form has attained a length of 65 mm. (83 mm.), 

 3 



