126 THE SEA-SCORPION-FAMILY. 



old the length is 8*4 mm., and the yolk has diminished, while 

 the oil-globule has been elevated to the gullet. 



The young forms were kept in the tanks till the 17th day, 

 but development proceeded slowly under the somewhat un- 

 favourable circumstances. On the 7th day they measured 

 7 '7 mm., and during the two or three subsequent days the 

 pigment made great progress, extending behind the vent, and 

 passing from the dorsum down the sides. The chief changes 

 are the straightening of the ventral line from the head to the 

 vent, from the diminution of the yolk, the disappearance of the 

 oil-globule, the increase in the length (forward) of the lower 

 jaw, and the presence of a distinct and broad pigment- 

 band on the side of the body a little behind the vent. The 

 notochord is still perfectly straight at the tip of the tail, and 

 the circulation is much as before. On the 10th day the yolk 

 had disappeared, but the embryonic fin-rays were present only 

 in the tail. The absence of food would, as Mr Holt suggests, 

 suffice to explain the slow progress, but not altogether, since, in 

 the open sea, specimens of 9'5 mm. are occasionally procured 

 in a similar condition, viz., having a membranous dorsal and 

 anal, and only embryonic rays in the tail. Those in confine- 

 ment differed in having no trace of the ventral fins, and the 

 thickening beneath the tail was better marked in the free forms ; 

 spines on the gill-cover were also present. 



In specimens of 7 mm. (in spirit) captured in the bottom- 

 net in the bay, the head has much increased in size, the fish is 

 thick-set, and the gill-cover has minute spines. Embryonic 

 rays are well developed in the tail, and a thickening occurs 

 beneath the central axis. The black pigment has largely 

 extended along the dorsum to a line behind the vent, and it is 

 more abundant on the head. Such a form contrasts with the 

 slender and ill-nourished specimens reared in the tanks, in 

 which the abdomen was shrunken and the end of the gut 

 distended, as if the vent were closed. The breast-fins, however, 

 were large. 



At the end of April and beginning of May, pelagic forms of 

 from 9'5 to 10 mm. are not uncommon at the surface, e.g., off 

 the Isle of May. The body has now considerably increased in 



