1010 



PISCES. 



in the cloacal cavity (s, s), these folds gradually 

 subside into a few simple pliaations. 



In some species ( Mustelus, Cuv.) of these 

 viviparous Sharks a very close attachment is 

 formed between the walls of the uterine portion 

 of the oviduct and the contained ovum, so much 

 so indeed as to remind the anatomist very for- 

 cibly of the placental connection that exists iu 

 the Mam mi Cera. In these, according to J. 

 JMiiller, the ovum, on its arrival in the oviduct, 

 is only covered with a kind of membranous in- 

 vestment or chorion, which is as thin and de- 

 licate as the amnion of Mammalia, and without 

 apparent organization. The sac which this 

 membrane forms is seven or eight times as long 

 as the vitellus, and its walls being regularly 

 plicated, are embraced by corresponding folds 

 of the lining membrane of the oviduct, so that 

 there is a very intimate adhesion between the 

 two. 



In the oviparous races of the plagiostome 

 cartilaginous Fishes the structure of the 

 oviduct is somewhat different, in order to pro- 

 vide for the formation of the egg-shell or horny 

 envelope wherein the egg is contained when 

 extruded from the body, the organization of 

 which is not a little curious. The glandular 

 portion of the oviduct is extremely thick, or 

 rather is enclosed in a dense glandular mass 

 (rudimental gland), the substance of which is 

 entirely made up of close-set transverse secern- 

 ing tubes, which pour their secretion into the 

 oviduct through innumerable orifices, which are 

 aggregated together in a part where the course 

 of the lining membrane of the oviducal canal 

 is interrupted, and free passage thus left for the 

 escape of the rudimental secretion, which, be- 

 coming thus deposited on the surface of the 

 egg, hardens into a tough horny substance, 

 which constitutes its external covering or egg- 

 shell. The shape of these eggs is remarkable ; 

 the ecjg-shell when completed resembles an ob- 

 long horny pillow-case, the four corners of 

 which are prolonged iuto tendril-like processes, 

 the use of which appears to be that they serve 

 as anchors by becoming interlaced with the 

 branches of submarine plants or ramose corals, 

 and thus preserve the egg and its delicate con- 

 tents from being washed away by the agitation 

 of the waves. From the tough coriaceous or 

 horny texture of these egg-shells, another pro- 

 vision becomes necessary, in order that the 

 mature embryo shall be enabled to escape from 

 confinement and enter upon an independent 

 existence. In the eggs of Birds this is abun- 

 dantly provided for by the brittle texture of the 

 calcareous substance in which they are en- 

 closed, allowing the chick to break its way out 

 of its fragile covering, a mode of egress which, 

 in the case before us, would evidently be im- 

 practicable. This difficulty is met by a very 

 beautiful contrivance. The horny walls of the 

 eggs of the plagiostome Fishes are continuous 

 all round, except at one extremity, where, to 

 use a homely illustration, the end of the pillow- 

 case remains unsewn, the edges of the slit thus 

 left being merely kept in apposition by the 

 elasticity of the horny envelope. By this ele- 

 gant arrangement all intrusion from without is 



effectually prevented, and at the same time, 

 seeing that the valves will separate on the ap- 

 plicatioji of a very slight pressure from within, 

 they soon yield to the efforts of the young fish 

 to escape from its cradle, and afterwards close 

 again so accurately that it is difficult, without 

 attentive examination, to detect the existence of 

 the fissure. 



As amongst mammiferous animals certain 

 races are provided with a marsupium or pouch, 

 in which their immature young are carried 

 about for a considerable period previous to 

 their birth, so do we find certain Fishes provided 



Fig. 539. 



yy- 



Viscera of Synijnathus acus(male^). 



a, liver ; b, communication between the swim- 

 ming bladder and the alimentary canal ; c, sto- 

 mach ; fl, intestine ; f, allantoid bladder ; g, yg, 

 testes ; m, kidney ; n, marsupial pouch ; o, o, cells 

 in interior of ditto. 



