SEXUAL CHARACTERS—ELASMOBRANCH FISHES 247 
outward) as to display the principal and accessory structures to 
best advantage. 
SCYLLIUM CATULUS 
The large spotted dogfish or nursehound 
Figure 1 shows the relative positions of the structures enu- 
merated in the introduction. The apopyle is so close to the cloaca 
cm. 
Fig. 1 Scyllium catulus. A., apopyle; H., hypopyle; S., siphon; P:, para- 
siphon; S.7., siphon tube; Cl.7., clasper tube; Cl.P., cloacal pouch; Rh., rhi- 
pidion. (On the observer’s right the siphon is shown actually inflated, the 
parasiphon diagrammatically so.) 
that the spermatozoa which are constantly oozing from that 
aperture, at any rate in dogfish caught during the first half of 
the year, cannot fail to enter it of their own propulsion; beside 
which they are also sucked into it by a down-draught from the 
siphon, as will presently be demonstrated by experiment. 
