THE ELASMOBRANCH FISHES 



805 



Squalus the gland is considerably increased in size and in Raja, Scyllium, and 

 Heterodontus francisci it becomes relatively of immense dimensions. If the 

 gland in Scyllium be taken as a type for study we find that it is divided into 

 a dorsal and a ventral half. A section through it shows that these halves are 

 divided into anterior and posterior areas; the former secretes albumen, the 

 latter the shell proper. The glands which are actively engaged in secreting the 

 shell are seen to advantage in figure 262. Here the folds are very high and the 

 secretory cells (t.gl.) at their bases are large. As the horny substance is formed 

 for the shell it passes into the cavity of the 

 shell gland which acts as a mold for the shell. 



TYPES OF EGG SHELLS 



Two types of shells are formed : the per- 

 manent and the temporary shells. In the per- 

 manent shell the young undergoes its de- 

 velopment to the form of the adult, after 

 w^hich it emerges {Scyllium^ Raja, Heiero- 

 dontus) . In both Scyllium and Raja the shell 

 is a rectangle, from the angles of which pro- 

 jections extend. These projections function 

 either as tendrils (Scyllium.) which coil 

 around solid objects and anchor the egg, or 

 they serve as spikes to fix the developing egg 

 in mud or sand flats (rays). Figure 264 

 shows the shell of the California swell shark, 

 Cephaloscyllium, which in all essential re- 

 spects is like that of the other Scyllidae ex- 

 cept that in the figure its tendrils appear 

 shorter. These tendrils are, however, long 



and are produced both from the upper and the lower angles. In color the ma- 

 ture shell is clear amber of equal shade throughout. Such shells of Cephalo- 

 scyllium, however, which are in the process of formation are whitish when first 

 removed from the oviducts; but these color w^ith age. 



In Heterodontus the shell is shaped like a screw with a characteristic double 

 flange extending from its apex to the large perforate end. The flange in Heter- 

 odontus francisci (fig. 263a) or H. philippi is broad and is thrown into four 

 or five coils. In Heterodontus galeatus (fig. 263b) the flanges are narrower, 

 and tendrils are present which may reach the extreme length of more than 

 seven feet (Waite, 1896). 



The attachment of the egg in egg-laying has been studied in Scyllium by 

 Kopsch (1897). It is found that as the egg passes through the cloaca its 

 tendrils, upon coming in contact with a solid, coil firmly around it, thus fixing 

 the egg in place (fig. 265). Two eggs are usually deposited at about the same 

 time and many are laid during the season. 



Fig. 265. Attachment of egg of 

 ScyUium. (After Kopsch.) 



