Gaily Marine Laboraiory ^ St. Anchews. 259 



Swarms of the earlj postlarval sea-scorpions, about 7 to 

 7"5 millini. (in spirit), are occasionally captured in the surface 

 tow-nets, as in the Forth, e. g. in March and April — indeed, 

 just after the absorption of the yolk. Such pelagic forms 

 have only embryonic rays in the tail-fin. The body and tail 

 are translucent, whilst the head and abdomen have a pale 

 greenish hue with black chromatophores, and the eyes have 

 a silvery lustre. A line of black pigment-specks runs along 

 the ventral edge of the muscle-plates behind the vent almost 

 to the tail. 



3Jr. Holt observed that the lower jaw is movable two days 

 after hatching and that the vent is open. In our examples a 

 thickening below the axis of the tail occurred. When six 

 days old the length is 8*4 millim. 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 seventeenth 

 day, but development proceeded slowly under the somewhat 

 unfavourable circumstances. On the seventh day and durino- 

 the two or three subsequent days the pigment made great 

 progress, extending behind the vent, and passing from the 

 dorsum down the sides. On the tenth 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 millim. are occasionally pro- 

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

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

 confinement, however, differed in having no trace of the 

 ventral fins, and the thickening beneath the tail was betrer 

 marked in the free forms, while spines on the gill-cover were 

 also present. 



In specimens of 7 millim. (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 thick- 

 ening occurs beneath the notochord. 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, for in the latter 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 millim. are not uncommon at the surface, e. g. 

 off the Isle of May. The body has now considerably in- 

 creased in bulk, a series of sharj) spines project from the gill- 

 cover and two on each side of tiie occiput. The larval tail is 



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