162 THE SPINY DOGFISH— THE SKATE 



Second Day 

 B. The Urino-genital System. 



1. The Female. 



Remove the pelvic girdle from between the bases of the pelvic fins, taking 

 care not to injure the structures lying beneath it. The mature ovary is a large 

 lobulated organ, suspended by a fold of peritoneum from the dorsal wall of the 

 body-cavity, and the eggs or ova form conspicuous prominences on its surface. 

 It then appears paired as the eggs lie on each side of the vertebral column. 

 The oviducts (MuUerian ducts) are paired and extend along the whole length of 

 the dorsal wall of the peritoneal cavity. Anteriorly they unite in front of the 

 liver (on the suspensory ligament), and communicate with the peritoneal cavity 

 by a common slit-like opening. In the anterior portion- of each oviduct a 

 swelling appears ; these are the oviducal or shell glands, which secrete the 

 horny egg-capsules ("Mermaid's purse"). Posterior to this the oviduct be- 

 comes enlarged and may contain encapsuled eggs or developing embryos 

 {Acanthias is viviparous, Scyllium is oviparous). The oviducts unite terminally 

 and open into the cloaca by a common opening, behind the anus. 



Cut open the cloaca and note the urinary papilla, situated in the middle of 

 the anterior wall, and the opening of the oviducts behind the anus, the opening 

 of the rectum. 



Remove the peritoneum from the dorsal wall of the body-cavity and note 

 the kidneys (opisthonephros), a pair of long narrow organs Ipng close to the 

 vertebral column, one on either side of it. The ureters (metanephric ducts) are 

 four to six fine tubes which issue from the posterior part (metanephros) of each 

 kidney and open separately into the urinary sinus. The urinary sinus is formed 

 by the union of the dilated ends of the Wolffian or mesonephric ducts, which are 

 rudimentary and straight in the female. [In the Skate (Raia) there are two ovaries.] 



Make a drawing of the female urino-genital system. 



2. The Male. 



Remove the pelvic girdle as in the female. Remove also the peritoneum 

 from the dorsal wall of the body-cavity so as to expose the kidneys, etc. 



The testes, when fully developed, are a pair of conspicuous elongate organs 

 attached to the roof of the body-cavity by a fold of the peritoneum. The 

 kidneys (opisthonephros) are larger than those of the female, and are differ- 

 entiated into two parts : the anterior part or mesonephros (Wolffian body) is 

 well developed and functions as the genital epidid}mais, and the posterior part 

 or metanephros is the kidney proper. The ureters are formed each from the 

 union of about five metanephric ducts, and open into the urino-genital sinus. 



From the anterior end of each testis arise delicate efferent ducts which pass 

 to the epididymis (mesonephros or Wolffian body) and are there connected with 

 the sinuous Wolffian duct which acts as the vas deferens. Posteriorly each vas 

 deferens expands to form an elongated and spindle-shaped vesicula seminalis, 

 which opens into the urino-genital sinus (i.e., the united bases of two sperm 

 sacs, which project forwards beneath the vesiculse seminales). 



The urino-genital sinus opens into the cloaca behind the anus or opening of 

 the rectum. 



