DEVELOl'MENT 3 r 



system of spines round the edges of the body, on the abdomen, on the caudal fin and 

 on the base of the pelvic fin, but there are none on the head (Fig. 21A). The arrange- 

 ment of these spines is more or less constant in a particular species, and provides a 

 useful character for distinguishing the post-larvae of the European species. The 

 spines gradually disappear as metamorphosis approaches, although some may be 

 retained to form spinules on the scales. In post-larvae identified by Kyle as Ancylop- 

 setta sp. the first six or seven rays of the dorsal fin are elongated and the edge of the 

 pra?opercu!um is provided with a strong spiny armature ; in addition, there is a strong 

 spine on each side of the dorsal fin in the sphenotic region of the head. Similar, but 

 rather more feeble, spines occur on the head in the post-larval Turbot and Brill 

 (Scophthalmus) etc. (Fig. 21c), and the skull itself may be provided with spinous 

 protuberances (" otocystic spines ") in post-larval Lepidorhombus , Phryiwrliombus 

 and Zeugopterus. All these structures disappear at or before the metamorphosis. 

 The post-larval teeth pre.sent in Ariioglossus, Bothus and other genera are of the same 

 nature as the spines described above, and appear to be cast off at about the same time. 

 An air-bladder is developed as a transient post-larval character in many Flatfishes. 

 According to Kyle (1921, p. 109), "sinistral forms have almost invariably an air- 

 bladder lying more or less on the left side . dextral forms have either no 

 air-bladder or the air-bladder more or less on right side." Among our own Flatfishes, 



Fig. 20. — Egg of .\, Brill (Scophthalmus rhombus) [after Mcintosh] ; b. Plaice {Plcuronectes 

 ptatessa) [after Ehrenbaura] ; c, Sole {Solea solea) [after Cunningham]. Greatly enlarged. 



those of the family Pleuronectids (Halibut, Dab, Lemon Sole, Witch, Flounder and 

 Plaice) have no trace of an air-bladder at any stage of their development. In the 

 sinistral Bothida-, on the other hand, this is generally present, but within the sub- 

 family Scophthalminae an air-bladder is present only in the post-larval Turbot and 

 Brill, and wanting in all the other genera. In the genus Solea an air-bladder is 

 present in post-larval S. solea and S. lascaris, but absent in S. variegata. 



An interesting feature of the development of Flatfishes is the length of the pelagic 

 larval and post-larval life, as this may have an important bearing on the distribution 

 of the species. In a general way, two types of larvae may be distinguished : those 

 with a very short pelagic life and early metamorphosis (e.g. Limanda, Platichthys, 

 Pleuronectes), and those in which the pelagic life is more prolonged (e.g. Arnoglossus, 

 Bothus). In Arnoglossus the metamorphosis is said to take place at the surface, and 

 it has been stated that the young stages of Bothus may lead a pelagic life for a short 

 time after the transformation has taken place (Kyle, 1913, p. 5). Generally speaking, 

 the Flatfishes of northern seas have a short pelagic life, and it is mostly among species 

 of tropical and subtropical regions that the metamorphosis is delayed. Frequently 

 post-larvje of some species of Bothus are captured in mid-ocean many miles from 

 land, often at considerable depths below the surface, and the " Thor ", " Dana ", 

 Discovery " and other vessels have collected large numbers of these larval forms 

 with the eyes still symmetrical in mid- Atlantic, in some cases hundreds of miles from 

 the nearest shore. In this genus the pelagic lar\'al life is very long. In B. podas. 



