THUKE-Sl'lNEU STICKLKBACK. 



(i4;t 



witliout rays. Tlii-' Nciitnil iiKU\i;iii in front of tlic ;iii;il 

 fill tonus M more ur k'ss distiiicl hrciik tor tlie veut; 

 anil in front of tiie vent, at a greater or less distance 

 therefroiu, lie the pelvic bones so cliaracteristie of the 

 Sticklebacks, or rather their covering bones. Here we 

 see the so-called pelvic bones ijiropclris) distinctly si- 

 tuated as osseous growths in the skin, in an homologous 

 series with the interhsvnial plates mentioned above; 

 while in the other Teleosts the pelvic bones, in their 

 position and often in their form as well, liave |)roved 

 quite as distint-tlv homologous with the interlnenial 

 bones (spines) themselves. This difference is aceomp;i- 

 nied b^- another: in the Sticklebacks the ventral tins 

 are attached to the middle (in Gastrtea) or to the an- 

 terior part (in Gasterosteus") of the so-called pelvic 

 bones, while in the other Teleosts the}' articulate with 

 the liind exti'emity of these bones. 



In the Three-spined Stickleback, as in the following 

 species, the ]3elvic bones, with their inner margins unit- 

 ed bv a suture in the median line of the belly, form 

 a compact, triangidar plate, pointed behind, \\hieh at 

 the anterior part of each of its sides sends out a foliate 

 disk up the sides of the belly. In the Three-spined 

 Stickleback this disk generally meets three of the lateral 

 jilates of the body on each side, thus forming a com- 

 plete pelvic girdle, belonging to the dermoskeleton. The 

 anterior margin of the pelvic plate is straight or only 

 slightly concave''. The two anterior corners touch the 

 interclavicles already mentioned, which in the Three- 

 spined Stickleback are comparatively broad — their 

 breadth at the middle about equal to the distance be- 

 tween them'' — and so long that the pelvic bones lie 

 entireh' behind the insertions of the pectoral tins. 



In the typical Three-spined Stickleback the sides 

 of the body are almost entirely covered bv a corslet 

 formed by a I'ow of jjlates belonging to the lateral line, 

 the only naked parts being the belly itself, up to a level 

 with the bottom of the insertions of the pectoral fins, 

 a narroAv strip along the soft-rayed dorsal and the anal 

 fins, and the patch in front of each of the pectoral tins. 



Among these lateral plates the middle ones, below or 

 just in front of the last s|)inous ra\ on the l)ack, are 

 the largest (deepest). For\vard and backwai-d the plates 

 diminish in size; but on the last eight or nine (on the 

 peduncle of the tail) we find a raised, median carina, 

 following the longitudinal dii'cction of the bod\-, and 

 perceptible, though not ver\' distinct, to the twelfth 

 plate, counting from behind. This median carina ren- 

 ders the breadth (thickness) of the body no less along 

 the greater part of the tail than it is in front; but just 

 in front of the caudal tin the breadth decreases mow or 

 less rapidly. The lateral line, which pierces these i)lates, 

 runs from the temporal region parallel to and near the 

 back. The sixth ])late, counting from in front, is the 

 most constant in ])osition, and unites the 4th dor.sal 

 plate to the top of the ascending disk of the pelvic 

 bones, with which disk the 5th and 7th lateral plates 

 are also contiguous in most cases. These three lateral 

 plates (5 — 7) are also the most persistent, while the 

 others may be I'educed, attenuated, and obliterated, as 

 we have described above. The first to disappear are 

 the plates that lie on the posterior jiart of the ventral 

 sides and the anterior part of the sides of the tail, 

 between the .seventh plate (from in front) and the ca- 

 rinated caudal plates : — a form of this description from 

 Norway has been named hemigymnus by Collett. Later 

 the fish loses both the foremost plates (in front of the 

 fifth) and the hindmost, carinated caudal plates, and 

 the peduncle of the tail undergoes more and moi"e la- 

 teral compression {gijmnurus). At the same time the 

 silvery lustre of the body generally disappeai's, and the 

 back develops more or less distinct, dark spots or trans- 

 verse bands, suggesting a retrogression to the juvenile 

 characters. 



The head is laterally compressed; the cheeks are 

 parallel; but the snout is attenuated in front even on 

 the sides. The upper and lower profiles of the head 

 meet at fairly equal angles (the upper sloping as much 

 as the lower rises), with the exception of a break formed 

 by the articulation of the lower jaw. Tins break is 



" In the North American Apeltes quadracus the pelvic houcs are separated from each other hehind, as in the Fifteeu-spincd Stickle- 

 back; the anterior part of these bones (in front of the ventral spines) is short, broad, and continuous, as in the Three-spined Stickleback, 

 but without any ascending disk at the lateral margins. The interclavicles are so small that the anterior part of the pelvic bones lies below 

 the insertion of the pectoral fins, with the ventral spines just behind it. 



'' In the variety islandicus (according to Sauvagk, !. c, p. "21. p). 1, fig. 8, '(), as in the above-mentioned (/^mHio-HS form from Green- 

 land, the pelvic plate is both narrow and deeply incised at the anterior margin, just as in the Tcn-spined Stickleback. 



' In some specimens of the above-mentioned f/i/ninuriin form from Greenland, however, the interclavicles are as narrow and as widely 

 separ.ited at the middle of tlicir lengUi as in the Ten-spined Stickleback. 



