The testicles are always paired, rounded, and situated in the anterior part of the abdominal cavity, covered by the liver. _Vasa efferentia_ pass the semen into a much-convoluted _epididymis_, which is continued into the _vas deferens_; this, at the commencement of its course, is spirally wound, but becomes straight behind, and has its end dilated into a seminal reservoir. It opens with the urethra in a papilla within the cloaca. The so-called _claspers_ of Chondropterygians (Fig. 78) are characteristic of all male individuals. They are semi-ossified appendages of the pubic, with which they are movably joined, and special muscles serve to regulate their movements. Sometimes they are armed with hook-like osseous excrescences (_Selache_). They are irregularly longitudinally convoluted, and, when closely ad-pressed to each other, form a canal open at their extremity. A gland, abundantly discharging a secretion during the season of propagation, is situated at, and opens into, the base of the canal. It is still doubtful whether the generally-adopted opinion that their function consists in holding the female during copulation is correct, or whether they are not rather an intromittent organ, the canal of which not only conducts the secretion of their proper gland but also the impregnating fluid. [Illustration: Fig. 79.--Egg of a Scyllium from Magelhan’s Straits (? Sc. chilense). Natural size.] [Illustration: Fig. 80.--Egg-shell of Cestracion philippi, half natural size, linear. I. External view. II. Vertical section. _a_, One spiral ridge; _b_, The other spiral ridge; _c_, Cavity for the ovum.] The ova of the oviparous Chondropterygians are large and few in number; they are successively impregnated, and the impregnation must take place before they are invested with a tough leathery envelope which would be impenetrable to the semen, that is, before they enter the uterus; therefore, copulation must take place in all these fishes. The form of the egg-shell differs in the various genera; generally (Fig. 79) they are flattened, quadrangular, with each of the four corners produced, and frequently prolonged into long filaments which serve for the attachment of the ova to other fixed objects. In _Notidanus_ the surfaces are crossed by numerous ridges. In _Cestracion_ (Fig. 80) the egg is pyriform, with two broad ridges or plates, wound edgewise round it, the two ridges forming five spires. The eggs of _Callorhynchus_ (Fig. 81) have received a protective resemblance to a broad-leaved fucus, forming a long depressed ellipse, with a plicated and fringed margin. [Illustration: Fig. 81.--Egg of Callorhynchus antarcticus. _a_, Cavity for the embryo.] CHAPTER XIII. GROWTH AND VARIATION OF FISHES.