Postlarval and Young Stages o/Oiius mustela. 135 



As the fish grows beyond this stage tlie tips of the fin-rays 

 are absorbed, and it is this which gives rise to the siiortenin;^ 

 of the fin (fig. 3). A transverse section through the anterior 

 portion of the fin is represented in fig. 5, four fin-rays being 

 cut. In this specimen the pigment is very well developed, a 

 complete layer of pigment-cells being present on the side 

 which is directed outwards. The bundles of nerve-fibres are 

 also shown in the section, as well as the undifferentiated 

 character of the epithelium. In the next stage gradual 

 differentiation of the epithelium takes place, accompanied by 

 absorption of the membrane between the first and second and 

 the second and third fin-rays. This is most noticeable in the 

 case of that between the first and second fin- rays. When 

 this process is finished the first ray is free for about one third 

 of its length and the second ray projects about the same 

 distance beyond the tip of the first. The fin-rays themselves 

 become thicker and the extremities are provided with sensory 

 organs. The pigment of the tips, which was so marked in 

 previous stages, is now reduced to a few cells of the usual 

 stellate type (fig. 4). At this stage the free tips are much 

 longer in proportion than in the adult, but as the fish grows 

 the fin becomes deeper and soon acquires the proportions 

 characteristic of the adult. 



Adult Jin. — Fig. G (PI. V.) represents the fin of a roekling 

 of 98 millim. which was obtained below stones in a rock-pool 

 at St. Andrews. At this stage the fish has all tlie characters 

 of the larger-sized aduhs. The fin-rays, of which there were 

 seven, vary in different cases from seven to nine. The former 

 number is more usual, the latter baving been only found in 

 one specimen. The last two or three rays are very short. 

 The tirst is separated from the second, which is the longest, 

 for a considerable distance. Over the surface of the fin are 

 scattered small brown pigment-cells, while near the base 

 occur larger black chromatophores which are visible as small 

 black dots when the fin is viewed through a lens. In trans- 

 verse section (tig. 7) the following structure is shown : — 

 Centrally are five pairs of dermal fin-rays (dr.), above and 

 below each of which lie a pair of nerve-cords (only five pairs 

 of fin-rays are shown, as the section is taken about the middle 

 of the tin, and so does not cut fin-rays G and 7). The 

 nerve-cords give off branches to supply the sense-organs, 

 dividing as they proceed towards the tips of the rays, which 

 are more richly innervated than the rest of the fin on 

 account of the greater imniber of sensory bodies there. The 

 rest of the space is filled up with loose connective tissue in 

 ■which ramify small blood-vessels. The epithelium consists 



