146 



HARDWICKE'S SCIENCE-GOSSIP. 



[July 1, 1S69. 



separated from each other, but nearer the circum- 

 ference they are more compressed and very firmly 

 adherent ; from the looser textures nucleated cells 

 (fig. 112) may be selected, which bear a remarkable 

 resemblauce to the cellular tissue of plants. 





Fig. ill. Hexagonal cells, 

 x 250 ; from the chorda 

 dorsalis of Petromyzon 

 murinus. 



Fig. 112. Nucleated Cartilage 

 cells, x 250 ; from the chorda 

 dorsalis of Petromyzon mari- 

 nus. 



The order Cyclostomata, according to Dr. Gray, 

 have the " gills saccate, body elongate, pectoral and 

 ventral fins none ;" and the family Petromyzonidee 

 (in contradistinction to the Myxinidce) have the 

 "nasal aperture closed, and the palate entirely 

 covered with skin." The generic characteristics of 

 the Lamprey, Petromyzon marimis, often called 

 the Sea-Lamprey, and by Cuvier la grande Lam- 

 proie, are thus defined by Yarrell : " Body smooth, 

 elongated, cylindrical like that of an eel; the head 

 rounded, the mouth circular, armed with hard tooth- 

 bke processes, the lip forming a continuous circle 

 round the mouth ; seven apertures on each' side of 

 the neck, leading to seven branchial cells ; no pec- 

 toral or ventral fins ; the skin towards the tail ex- 

 tending in a fold from the body both above and 

 below, forms dorsal and caudal fins," to which may 

 be added that both dorsal fins are convex, that the 

 first dorsal fin is placed behind the middle of the 

 oody, and is lower and shorter than the second ; the 

 second appears at first sight to be directly con- 

 tinuous with the caudal, but the true anatomical 

 limit of the former is indicated by a deep notch, 

 and on close examination it will be seen that the 

 posterior extremity of the dorsal is superimposed 

 over the first rays of the caudal, while a kind of 

 crease is observable in the skin, running from the 

 notch forwards in an oblique direction below the 

 dorsal : from the notch the true caudal fin extends 

 backwards, surrounding the tail ; having reached 

 the inferior aspect, it gradually loses itself in the 

 skin of the abdomen without constituting a true 

 anal fin. 



The skin is perfectly smooth ; the colour of the 

 body is olive-brown, mottled and spotted on the 

 back and sides with darker green and dark brown 

 — hence it is sometimes called P. macidahis, — the 

 margins of the fins inclining to reddish brown ; but 

 these colours seem to vary, being modified probably 

 by local conditions. Of two fine specimens received 

 from the Severn within the last fortnight, the one 



abounded in golden tints, while the spots were 

 almost blue. 



Fig. 113. Pigment-cells, x 250, from the skin of P. marinus. 



The head has a somewhat elongated appearance 

 when the mouth is closed; the latter is generally 

 spoken of as a longitudinal slit on the under side, 

 but it is rather buttonhole-shaped, with a remark- 

 able commissural fold admitting of great expansion 

 at the posterior angle ; the lips are soft and tumes- 

 cent ; when the mouth is opened wide, it becomes, 

 as Mr. Couch aptly describes it, " circular and ter- 

 minal, so that the fish appears as if the head had 

 been cut off;" it forms a most efficient suctorial 

 apparatus ; the flexible cartilaginous rim representing 

 the jaws adapts itself to the form of the surface to 

 which the mouth is applied, and on the motile piston- 

 like tongue being retracted a vacuum is formed, and 

 a hold of extraordinary power is obtained. By 

 means of this mechanism, the various genera of the 

 family Petromyzonidce affix themselves to the rocks 

 and stones at the bottom when in a state of repose. 

 I have seen the bed of the Severn, in certain favour- 

 ite spots between Gloucester and Tewkesbury, 

 literally black with thousands of " stone-suckers ;" 

 safely moored by the head, with their tails floating 

 free, swaying from side to side in the swift current 

 of the deep dark river, they looked so like the 

 waving tangle of the waterweed, that they would 

 certainly have escaped the observation of any 

 person who was not specially bent upon discovering 

 them. In still waters they swim with a lateral un- 

 dulating motion, but in rapid streams they advance 

 by a succession of plunges, fastening themselves at 

 the end of each spot to any convenient object. 



The armament of the mouth renders it a most 

 formidable weapon of offence. This peculiar dental 

 apparatus is thus described in Owen's "Odonto- 

 graphy":— 



" In the Lampreys there are labial and maxillary 

 as well as palatal and lingual teeth ; all these are 

 horny substances of a simple, conical, sharp-pointed 

 form ; they are hollow and supported on conical 

 reproductive pulps. The pulps of the teeth are 

 firmly attached by their base to the fibrous tissue 

 of the lining membrane of the lip. The labial teeth 

 of the outer or marginal circle are the smallest ; 

 from these the teeth increase in size as they ap- 



