320 THE DOG-FISH 



eggs are passed into the cavity of the outer gill of the 

 female, where they undergo the early stages of their develop- 

 ment. The larva of the fresh-water mussel is a peculiar 

 bivalved form, very unlike the adult, and called a glochidium^ 

 but in the more typical molluscs the embryo leaves the egg 

 as a trochosphere, closely resembling that of Polygordius. 



THE Doc-FisH. 1 



A dog-fish is bilaterally symmetrical, the nearly cylin- 

 drical body (Fig. 80, A) terminating in front in a blunt 

 snout and behind passing insensibly into an upturned tail. 

 Externally there is no appearance of segmentation. 



The mouth (MtK) is on the ventral surface of the head 

 or anterior region of the body ; it is transversely elongated, 

 and is supported by jaws which are respectively anterior 

 (upper) and posterior (lower). They thus differ funda- 

 mentally from the jaws of arthropods, which are modified 

 appendages and are therefore disposed right and left. 



A short distance behind the mouth are five vertical slits 

 (B, Ext. br. ap) arranged in a longitudinal series, the 

 external branchial apertures or gill-clefts. The vent, or 

 cloacal aperture (An) is situated on the ventral surface a 

 considerable distance from the end of the tail. That part 

 of the body lying in front of the last gill-cleft is counted as 

 the head, all behind the vent as the tail, the intermediate 

 portion as the trunk. 



1 For a detailed description of a dog-fish see Marshall and Hurst, 

 Practical Zoology (London, 1888), p. 196. For descriptions of other 

 fishes, equally suitable in some respects as types of Vertebrata, see 

 Rolleston and Jackson, Forms of Animal Life (Oxford, 1888), pp. 83 

 and 273 : and Parker, Zootomy (London, 1884), pp. I, 27, 86. 



