988 



PISCES. 



through the exposed apertures. There is, more- 

 over, another remarkable arrangement in the 

 Lampreys, whereby, notwithstanding the sucto- 

 rial character of their mouths, the members of 

 this genus have the character of their respiratory 

 apparatus approximated to that of other races 

 of Fishes. This consists in the presence of a 

 small tubular orifice situated in the middle of 

 the back of the head, just in front of the eyes, 

 which leads downwards into the pharynx (b,) 

 into which it opens by the orifice c, so that 

 water can enter by this passage while the 

 mouth is kept immoveably fixed to the surface 

 whereunto the Lamprey has attached itself. 



The respiratory system of Branchiostoma is 

 highly curious in its arrangement. It may be 

 said to consist of two portions, a hyoid appa- 

 ratus of a very remarkable kind, and a respi- 

 ratory or branchial sac enclosed by a series of 

 thoracic ribs, both of which will require a par- 

 ticular description before we proceed further 

 vvitli this part of our subject. 



Hyoid apparatus. The hyoid apparatus 

 supports the mouth and guards its entrance, 

 the oral aperture, which is in the form of a lon- 

 gitudinal slit, being bounded on each side by a 

 series of hyoid cartilages, each of which consists 

 of several pieces articulated together so as to 

 form a continuous chain, which gradually 

 diminishes in thickness towards its anterior 

 extremity. To every one of these pieces is 

 attached a cartilaginous arch composed of two 

 cartilaginous stems, which are armed with 

 delicate denticulations, and covered with a fine 

 membrane, which is extended along the base of 

 the posterior ones like a thin interdigital web, 

 which forms the margin of the mouth. 



Branchial cavity. This is situated behind 

 the hyoid apparatus, and consists of a kind of 

 thorax composed of upwards of a hundred 

 transparent, cartilaginous, and hair-like arches, 

 the upper extremities of which are fixed in two 

 streaks of a soft white substance, which runs 

 along on each side of the inferior surface of the 

 chorda dorsal is on the sides of the inferior lon- 

 gitudinal ligament. The inferior extremities 

 of the ribs terminate in a more complicated 

 manner. Each alternate rib bifurcates. The 

 inferior branch on each side meets its fellow of 

 the opposite side at an angle in the median 

 line, while the superior branch likewise curves 

 up and meets the corresponding division of its 

 opposite, while the non-bifurcated ribs, which 

 are shorter, terminate in a line with the bifur- 

 cation of the neighbouring pairs. There are, 

 moreover, slender cross pieces joining the 

 neighbouring ribs together, disposed in an 

 alternating series like the joinings of stones 

 in masonry. The heart is situated at the 

 posterior extremity of the branchial sac be- 

 tween its posterior extremity and the com- 

 mencement of the liver; it consists, accord- 

 ing to Costa, of two delicate cavities situated 

 transversely above and a little to the right of the 

 intestine. These cavities (" orecchiette") com- 

 municate, and the blood passes from one to the 

 other, being driven forward by .rapid pulsa- 

 tions. From the second cavity of the heart 

 the blood is driven into the trunk of the bran- 



chial artery, which is situated between the in- 

 testine and the point of attachment between 

 the branchial ribs and the spinal column. The 

 course of the blood is at first rapid and very 

 distinct, owing to the size of the branchial 

 artery being considerable ; but as it runs for- 

 wards, it expends itself in numerous spiral ves- 

 sels, which, accompanying all the thoracic arches 

 and their transverse attachments, thus present 

 a considerable surface and constitute the bran- 

 chiae of the fish. Finally, towards the anterior 

 extremity of the respiratory cavity the branchial 

 artery has become so much reduced in size as 

 to be no longer distinguishable. 



In addition to the extensive surface furnished 

 by the respiratory sac there are three vascular 

 lamellae situated on each side of the fauces, 

 to which Costa applies the name of opercular 

 branchiae, believing them to be analogous to the 

 branchiae attached to the opercula of the Stur- 

 geon and other Fishes; these receive their sup- 

 ply of blood from an arterial trunk that sur- 

 rounds the back part of the roof of the mouth. 



Heart. The heart of Fishes (>g.522, C, A) 

 is situated beneath the throat immediately be- 

 hind the inferior terminations of the branchial 

 arches, where it is lodged in a compartment 

 partitioned off from the cavity of the abdomen 

 by a strong tendinous septum, which forms a 

 sort of immoveable diaphragm. In the Lam- 

 preys the heart is moreover encased in a cartila- 

 ginous capsule/ig.521, ?) formed by the poste- 

 rior portion of the cartilaginous frame-work that 

 surrounds the branchial apparatus. 



In the course of the circulation the heart of 

 Fishes is interposed between the systemic 

 veins and the organs of respiration, its office 

 being to propel the venous blood received 

 through the venae cavae into the gills, whence it 

 is conveyed by the arterial system to be distri- 

 buted to the body without the intervention of 

 any contractile cavity devoted to its propulsion 

 through the arterial trunks. For a diagram of 

 the course of the circulation in Fishes the 

 reader is referred to Jig. 319, vol. i. 



The heart of a fish is therefore at once distin- 

 guishable from that of any other vertebrate ani- 

 mal by the circumstance of its consisting of only 

 two cavities, an auricleand a ventricle, the former 

 of which receives the blood from the body, while 

 the latter diffuses it over the branchial surfaces. 

 The auricle (fig. 522, C) is very capacious, 

 having its walls made up of intercrossing mus- 

 cular fibres, which from their frequent decussa- 

 tions present internally a reticulate appearance. 

 It receives the venous blood from a sinus (D), 

 formed by the conjoined cavse through a single 

 orifice, which is guarded by two large semilu- 

 nar valves, arranged so as to prevent the reflux 

 of blood into the veins. 



The ventricle is considerably smaller than 

 the auricular cavity, with which it communi- 

 cates by a wide lateral orifice. Its shape in 

 the osseous Fishes is most usually somewhat 

 four-sided, and its muscular walls, considerably 

 thicker than those of the auricle, present inter- 

 nally a strung fasciculated appearance. Theau- 

 riculo-ventricular opening is generally guarded 

 by two strong valves ordinarily of a semilunar 



